user86: Along with that, I'd like to know what the highest rating of any human has ever been - including historical estimates.

user7500: According to this source, Computer Chess Champion Komodo has a rating of 3344 in 40/40.

user70: This page has a lot of good info.  Kasparov is the highest at 2851.  However, such comparisons are fraught with peril as ratings inflate, and they really can't be compared from era to era.  I don't think it will be long before Magnus Carlsen takes the 'highest rating' crown. This won't mean he's stronger than Kasparov at his peak.
Computer ratings are trickier.  Rybka 3 claims over 3200 Elo.  If this is the true, then only about 6 GMs in the world have a statistical chance of defeating it.
EDIT - that 3200 rating is on specialized hardware.  I think on a common (but beefy) PC, the rating is about 100 pts lower.  This means that currently there are about 30 players in the world who have a statistical chance of not losing.  None of them have an appreciable chance of winning.  Don't be depressed - no human has an appreciable chance of beating a car at a race.  Somehow this didn't make track and field less interesting.
EDIT in 2022:
Stockfish is now at 3546

86: <p>Along with that, I'd like to know what the highest rating of any human has ever been - including historical estimates.</p>
 7500: <p>According to <a href="http://www.computerchess.org.uk/ccrl/4040/" rel="nofollow">this source</a>, Computer Chess Champion Komodo has a rating of 3344 in 40/40.</p>
 70: <p><a href="http://en.wikipedia.org/wiki/Comparison_of_top_chess_players_throughout_history" rel="nofollow noreferrer">This page</a> has a lot of good info.  Kasparov is the highest at 2851.  However, such comparisons are fraught with peril as ratings inflate, and they really can't be compared from era to era.  I don't think it will be long before Magnus Carlsen takes the 'highest rating' crown. This won't mean he's stronger than Kasparov at his peak.</p>
<p>Computer ratings are trickier.  Rybka 3 claims over 3200 Elo.  If this is the true, then only about 6 GMs in the world have a statistical chance of defeating it.</p>
<p>EDIT - that 3200 rating is on specialized hardware.  I think on a common (but beefy) PC, the rating is about 100 pts lower.  This means that currently there are about 30 players in the world who have a statistical chance of <em>not losing</em>.  <em>None</em> of them have an appreciable chance of winning.  Don't be depressed - no human has an appreciable chance of beating a car at a race.  Somehow this didn't make track and field less interesting.</p>
<p>EDIT in 2022:
<a href="https://www.ichess.net/blog/best-chess-engines/" rel="nofollow noreferrer">Stockfish is now at 3546</a></p>
