user520: I'm using GNUChess with the Knights client (Knights doesn't seem to be THAT mature; crashes are common). Is there any equivalent of Chessmaster on Linux? What about good clients to play on servers like FICS.
I currently use Jin to play on FICS. But I think Jin doesn't expose all the functionalities of the server, e.g. chat isn't there in Jin. 

user9030: All top level chess engines like Komodo, Houdini, Rybka, Stockfish etc. are following UCI standards, which is platform-independent. It doesn't matter Linux, Windows, OSX or whatever. 
What you need(and what you probably ask) is a GUI for running engine. For this, I can suggest ChessX on Linux.
By the way, Chessmaster's engine is pretty bad comparing the engines above, I would never suggest to use it, if you are serious in chess.

user70: On the Mac, I use SCID as the interface and the Stockfish engine.  I've used Crafty and found it to be safe and effective if taken as directed.
There are probably Unix command-line versions for both, if that's what you're looking for.
EDIT - Since I wrote the above I've spun up a Windows box. I'm running Stockfish and Arena.  I find Arena annoying but better than Tarrasch.

user780: I like Xboard. Its a really cool interface for chess engines like GNUChess and it supports connecting to servers like FICS.  It can support chess variant engines too (including international variants).  It can be used to play Fruit Engine, (as another answer suggests.)
Here is a quote from its website:

XBoard is a graphical user interface for chess in all its major forms, including international chess, xiangqi (Chinese chess), shogi (Japanese chess) and Makruk, in addition to many minor variants such as Losers Chess, Crazyhouse, Chess960 and Capablanca Chess. It displays a chessboard on the screen, accepts moves made with the mouse, and loads and saves games in Portable Game Notation (PGN)  

It also has a Windows port.

user721: Stockfish is the strongest engine for linux AFAIK. The best FICS interface is BabasChess which is a Windows program but runs fine under Linux with Wine - i'm using it in OpenSuse 11.4 without any problems.
Another possibility is to use VirtualBox (use the rpm version from site and not the opensource version from linux repositories !) and run Windows as virtual machine with your favourite chess clients - Windows XP is sufficiant. I'm running Fritz (http://www.playchess.com/, which has a free client also => http://www.fritzhelp.com/download/PlayChessV6Setup.exe), and ChessPlanet client from the russian ChessServer of Convekta (ChessAssistant), also used Dasher Client from ICC.
-- EDIT after comment by Saibot --
You're confusing a specification (= protocol) with a concrete piece of software
built / implemented for a specific operating system.
The UCI protocol (UCI = universal chess interface) only describes the communication
of a chess engine and a chess user interface.
The Stockfish Linux version won't run on a windows box, just as  the Stockfish Windows
version won't run on Linux, whereas both support the UCI protocol.
Some engines - f.e. Stockfish - provide multiple implementations for different operating systems,
but many engines are for windows only.
So it's nothing wrong with best engine for Linux, best engine for Windows !
Engines implemented in a cross-platform language as Java ("write once run everywhere") will run on
several platforms, if Java VM is available for that operating system.

user800: For professional level engines, I run Houdini 1.5 (I imagine newer, paid versions of Houdini work as well) and Stockfish in SCID. The Houdini exe loads fine in SCID so long as wine is installed and the exe has been made executable. Both work very well for me, but I find Houdini's evaluations (e.g. -0.71 is a clear advantage to Black) to be the most meaningful. Stockfish evaluations are often inflated/optimistic (-0.71 might only mean a minuscule advantage to Black or +1.5 might not mean White is winning, but that Black in fact has strong compensation for the exchange. In my opinion, Houdini is particularly good at evaluating compensation for material deficit. I've tried the Toga, Fruit, and Glaurang engines; they are fine and , to my recollection, work well with SCID, but not as strong as Houdini and Stockfish.
For a simple chessmaster equivalent program, pychess (in the Ubuntu software center) might be suitable.
I also use Jin and Dasher in wine for ICC. I haven't tried FICS much.

user516: Engines:

Toga II (Package).
Glaurung (Package).
Fruit (Package).

All three are installable via Ubuntu Software Center and all three are UCI compliant (ie. so runs in Chessbase Fritz, Chess for Android, and/or Arena GUI). Arena is used with FICS so you should be able to use all 3.
Except for Glaurung I used both on occasion and was very satisfied with them.  
Installation:
sudo apt-get install toga2 glaurung fruit


520: <p>I'm using GNUChess with the Knights client (Knights doesn't seem to be THAT mature; crashes are common). Is there any equivalent of Chessmaster on Linux? What about good clients to play on servers like <a href="http://www.freechess.org/" rel="noreferrer">FICS</a>.</p>

<p>I currently use Jin to play on FICS. But I think Jin doesn't expose all the functionalities of the server, e.g. chat isn't there in Jin. </p>
 9030: <p><strong>All top level chess engines</strong> like Komodo, Houdini, Rybka, Stockfish etc. are following <strong>UCI</strong> standards, which is <strong>platform-independent</strong>. It doesn't matter Linux, Windows, OSX or whatever. </p>

<p><strong>What you need</strong>(and what you probably ask) is a <strong>GUI</strong> for running engine. For this, I can suggest <strong>ChessX on Linux</strong>.</p>

<p>By the way, Chessmaster's engine <a href="http://www.computerchess.org.uk/ccrl/4040/" rel="nofollow">is pretty bad comparing the engines</a> above, I would never suggest to use it, if you are serious in chess.</p>
 70: <p>On the Mac, I use SCID as the interface and the Stockfish engine.  I've used Crafty and found it to be safe and effective if taken as directed.</p>

<p>There are probably Unix command-line versions for both, if that's what you're looking for.</p>

<p>EDIT - Since I wrote the above I've spun up a Windows box. I'm running Stockfish and Arena.  I find Arena annoying but better than Tarrasch.</p>
 780: <p>I like <a href="http://www.gnu.org/software/xboard/">Xboard</a>. Its a really cool interface for chess engines like GNUChess and it supports connecting to servers like FICS.  It can support chess variant engines too (including international variants).  It can be used to play Fruit Engine, (as another answer suggests.)</p>

<p>Here is a quote from its website:</p>

<blockquote>
  <p>XBoard is a graphical user interface for chess in all its major forms, including international chess, xiangqi (Chinese chess), shogi (Japanese chess) and Makruk, in addition to many minor variants such as Losers Chess, Crazyhouse, Chess960 and Capablanca Chess. It displays a chessboard on the screen, accepts moves made with the mouse, and loads and saves games in Portable Game Notation (PGN)  </p>
</blockquote>

<p>It also has a Windows port.</p>
 721: <p><strong><a href="http://stockfishchess.org/" rel="nofollow">Stockfish</a></strong> is the strongest engine for linux AFAIK. The best FICS interface is <strong><a href="http://www.babaschess.net/" rel="nofollow">BabasChess</a></strong> which is a Windows program but runs fine under Linux with <strong><a href="http://www.winehq.org/" rel="nofollow">Wine</a></strong> - i'm using it in OpenSuse 11.4 without any problems.<br>
Another possibility is to use <strong><a href="https://www.virtualbox.org/" rel="nofollow">VirtualBox</a></strong> (use the rpm version from site and not the opensource version from linux repositories !) and run Windows as virtual machine with your favourite chess clients - Windows XP is sufficiant. I'm running Fritz (<a href="http://www.playchess.com/" rel="nofollow">http://www.playchess.com/</a>, which has a free client also => <a href="http://www.fritzhelp.com/download/PlayChessV6Setup.exe" rel="nofollow">http://www.fritzhelp.com/download/PlayChessV6Setup.exe</a>), and <strong><a href="http://ctart.chessok.com/pages/game-zone" rel="nofollow">ChessPlanet</a></strong> client from the russian ChessServer of <strong><a href="http://chessok.com/" rel="nofollow">Convekta</a></strong> (ChessAssistant), also used <strong><a href="http://www.chessclub.com/download-software" rel="nofollow">Dasher</a></strong> Client from <strong>ICC</strong>.
<br>-- <strong>EDIT</strong> after comment by Saibot --<br>
You're confusing a specification (= protocol) with a concrete piece of software
built / implemented for a specific operating system.
The UCI protocol (UCI = universal chess interface) only describes the communication
of a chess engine and a chess user interface.<br>
The Stockfish Linux version won't run on a windows box, just as  the Stockfish Windows
version won't run on Linux, whereas both support the UCI protocol.
Some engines - f.e. Stockfish - provide multiple implementations for different operating systems,
but many engines are for windows only.<br>
<strong><em>So it's nothing wrong with best engine for Linux, best engine for Windows !</em></strong><br>
Engines implemented in a cross-platform language as Java ("write once run everywhere") will run on
several platforms, if Java VM is available for that operating system.</p>
 800: <p>For professional level engines, I run Houdini 1.5 (I imagine newer, paid versions of Houdini work as well) and Stockfish in SCID. The Houdini exe loads fine in SCID so long as wine is installed and the exe has been made executable. Both work very well for me, but I find Houdini's evaluations (e.g. -0.71 is a clear advantage to Black) to be the most meaningful. Stockfish evaluations are often inflated/optimistic (-0.71 might only mean a minuscule advantage to Black or +1.5 might not mean White is winning, but that Black in fact has strong compensation for the exchange. In my opinion, Houdini is particularly good at evaluating compensation for material deficit. I've tried the Toga, Fruit, and Glaurang engines; they are fine and , to my recollection, work well with SCID, but not as strong as Houdini and Stockfish.</p>

<p>For a simple chessmaster equivalent program, pychess (in the Ubuntu software center) might be suitable.</p>

<p>I also use Jin and Dasher in wine for ICC. I haven't tried FICS much.</p>
 516: <p>Engines:</p>

<ul>
<li>Toga II (<a href="http://packages.ubuntu.com/precise/toga2">Package</a>).</li>
<li>Glaurung (<a href="http://packages.ubuntu.com/precise/glaurung">Package</a>).</li>
<li><a href="http://www.fruitchess.com/purchase.htm">Fruit</a> (<a href="http://packages.ubuntu.com/precise/fruit">Package</a>).</li>
</ul>

<p>All three are installable via Ubuntu Software Center and all three are UCI compliant (ie. so runs in Chessbase Fritz, Chess for Android, and/or Arena GUI). Arena is used with FICS so you should be able to use all 3.</p>

<p>Except for Glaurung I used both on occasion and was very satisfied with them.  </p>

<p>Installation:</p>

<pre><code>sudo apt-get install toga2 glaurung fruit
</code></pre>
