KGS http://www.gokgs.com/ IMO the best server for lower level games.
It as an active English speaking community Ranking system is relatively meaningful, even in the lower ranks
Many features in the client, such as game reviews
boardgames.stackexchange.com/users/854/CodesInChaos
IGS http://www.pandanet-igs.com/ ) IGS has a lot of players, but going up the rank ladder can also be quite time consuming.
Anyway, if you have an idea about your rank, you can set it up when you sing up.
Their english resources have been enhanced in the last year, with a social web and a new Java-based client that's actually quite usable (including an opponent search function).
Games tend to be serious and it lacks the communication facilities that you can find on KGS, but it's a good place to learn and it also has quite strong players.
There are some robots on the ranked games room to help you get a rank if you start at BC level (beginner class).
boardgames.stackexchange.com/users/None/
Dragon Go Server (DGS)
For turn-based playing I highly recommend Dragon Go Server , where I play all the time, too.
Simple and clean interface Private notes (very handy
if you are playing lots of turn-based go games at once)
There is a client for Android and iOS and a Windows notifier "Fine" rating (kyu/dan level with +- 50%) with "proper komi feature" to fine tune komi based on fine rating.
Its rating graph (shows rating reliability, has a Games/Time switch on X axis)
I know Gōng Yuán Chéng already included a link to Turn-based go servers on the Sensei's Library, and DGS is the first on that list, but DGS should have its own special place on this list.
More on DGS here: http://senseis.xmp.net/?DragonGoServer
boardgames.stackexchange.com/users/None/
OGS online-go.com is an excellent server if you're interested in turn-based go.
The best thing about the site is the tournament system.
There are five annual tournaments with multiple rounds in each category - even games, handicap games, and 9x9.
Some tournaments require you to win your group to advance, and some allow all players to advance, and then be matched up according to number of wins.
boardgames.stackexchange.com/users/2086/TimK
GoShrine The GoShrine is a good alternative to other servers: No download/install needed No Java/Flash requirement Play in the browser simply Levels similar to those on kgs Possibility to play bots as well
It's where I play when I use my Android tablet.
One disadvantage is there are too few players online at all time.
And of course you can review the links on these pages
boardgames.stackexchange.com/users/2096/龚元程
Tygem http://www.tygembaduk.com/
One of the most popular Korean servers and well known in the West for its large player base.
It was difficult to sign up to Tygem years ago, but they recently introduced an English client and website, solving this issue.
Players range from super weak to super strong (several top pros play on Tygem).
Their style tends to be more agressive.
There are noticeably many sandbaggers and the ranking system isn't optimal in the weaker ranks.
boardgames.stackexchange.com/users/854/CodesInChaos
WBaduk / CyberOro http://www.wbaduk.com/ Similar properties compared to Tygem.
But I haven't played much there.
boardgames.stackexchange.com/users/854/CodesInChaos
DashN http://club.dashn.com/english DashN is a Korean go server with an English interface.
It is also known as DashBaduk or Dash & Dot.
DashN and wBaduk are the only major Korean Go servers that offer clients with an English interface.
The common belief is that this is the place to play fighting games.
Tactics seem to also suggest a strong emphasis on territorial reduction.
( Sensei's )
boardgames.stackexchange.com/users/60/mafu
Yahoo Games
I'm surprised no one's mentioned that Yahoo has Go - there's consistently around 100~200 people on.
It's a good place for English beginners to play, and there's even some higher-rated players if you're up for a challenge.
boardgames.stackexchange.com/users/175/BlueRaja - Danny Pflughoeft
GOmatches.com www.gomatches.com
Great turn-based GO server.
It's completely web-based
so you don't need to download anything.
boardgames.stackexchange.com/users/3242/Markus
FlyOrDie
Sometimes I play here
You can play on browser without registration, the only one requirement is java...
Actually I do not know if it's good, but for a fast match it works well...
boardgames.stackexchange.com/users/None/
If you can get a beta key (they give out 200 every time the counter runs down on their site)
kaya.gs is a good site.
They're working on doing everything that KGS does wrong right.
I've played a few games there and I must say, its very cool.
You should give it a try or look on their FAQ if you have a minute.
boardgames.stackexchange.com/users/3250/MentatOfDune
http://www.usgo.org/go-internet
List of both Real Time Servers , as well as Turn-Based Servers (which I heard are recommended for beginners because of no time-limit).
Some of the sites mentioned in answers here might be found on this list.
I haven't really tried many of the sites mentioned on the list, because I'm just beginning my adventure with Go.
So I can't recommend any of the available links for now.
boardgames.stackexchange.com/users/2500/beam022
I used to play regularly on KGS but have been inactive for several years. I am frustrated by the Catch-22 of the KGS system: in order to play rated games, you must have a solid rating. Solid ratings come from playing many rated games.  Does anyone have advice for an enthusiast who wants to play competitively online? Where should I go and how do I establish a rank?
Where to play Go online?
boardgames.stackexchange.com
<online><go>
boardgames.stackexchange.com/questions/5474
boardgames.stackexchange.com/users/None/
There are a lot servers for going up the rank ladder: IGS, KGS, Dragon Go Server, OGS, DashN and Yahoo. http://www.gokgs.com is a recommended server for lower level games.  http://www.pandanet-igs.com has a lot of players, and for turn-based playing Dragon Go Server is also recommended. online-go.com  is another excellent server if you're interested in turn-based go.  http://club.dashn.com/english  is a Korean go server with an English interface.  And Yahoo  has Go , where there are consistently around 100~200 people on.  
http://www.gokgs.com is a recommended server for lower level games.  http://www.pandanet-igs.com has a lot of players, and for turn-based playing Dragon Go Server is also recommended. online-go.com  is another excellent server if you're interested in turn-based go.  http://club.dashn.com/english  is a Korean go server with an English interface.  And Yahoo  has Go , where there are consistently around 100~200 people on.  