I use Houdini 1.5, it's free, fast, and like JoseO said one of the strongest if not the strongest.
It is a bit silly though, what's the difference to me between a 3100 and 3200 program? Or for that matter a 2500 program? I don't know, but it's fun to have a really strong program 
I'll look into houdini
Stockfish: http://stockfishchess.com/
Houdini: http://www.cruxis.com/chess/houdini.htm
Fire: http://www.chesslogik.com/index.htm
These play better then you or I can ever hope to play. All free, 2 out of 3 open source. Pick one.
This is kind of a strange outcome. So, for some, Rybka's alleged plagiarism has introduced a new ethics to chess: Not using an engine that does not have an original code. Even though the engine has proven itself quite capable, and the matter appears to be which manufacturer will benefit monetarily from the product, some people will apparently no longer use the engine because of this. Hmmm...
This is kind of a strange outcome. So, for some, Rybka's alleged plagiarism has introduced a new ethics to chess: Not using an engine that does not have an original code. Even though the engine has proven itself quite capable, and the matter appears to be which manufacturer will benefit monetarily from the product, some people will apparently no longer use the engine because of this. Hmmm...
Yeah, it's dumb. He was kicked out of that computer champ camp, not convicted of a felony or anything. He still put in a lot of work for a good product. If he makes a super-strong Rybka 5 I'll certainly consider getting it...
even though I hate to spend money to upgrade from ludicrously super strong engine to ludicrously super strong engine +1 heh.
fritz 5.32 free
good enough to believe in and easyer to try to understand, so not like houdine or other 2800 engine.
Houdini, Firebird and Stockfish are all free and nothing personal but with a rating of 1308 it doesn't make a dam bit of difference which engine you use, including shredder, rybka or fritz. I'm rated 2066 here they're all light years ahead of my chess-playing ability (or lack thereof) as well.
Yeah, it's dumb. He was kicked out of that computer champ camp, not convicted of a felony or anything. He still put in a lot of work for a good product. If he makes a super-strong Rybka 5 I'll certainly consider getting it...
even though I hate to spend money to upgrade from ludicrously super strong engine to ludicrously super strong engine +1 heh.
He also violated the licenses under which both Fruit and Crafty are made available. Fruit is under the GPL, which legally requires that modifications or improvements also be released as free and open source under the GPL. Crafty's license quite explicitly prohibits unauthorized commerical use, and certainly Rasjik made a lot of money from Rybka as a commercial engine. So yes, he did break the law here. Some people do not wish to support or reward criminal activity. Evidently such concerns are irrelevant to you.
To be honest, yes.
Was it lawfully wrong? Yes. Was it morally wrong? No.
It's funny to say he stole open source code
I think the point is he used it to make money right? Victimless crimes don't impress me too much.
Anyway we could go back and forth on this quite a lot and I suspect it would be less of a discussion than a shouting match.
I mean... he's not totally clean morally... but he's not dirty enough that I'd boycott Rybka either :)
It's funny to say he stole open source code
I think the point is he used it to make money right? Victimless crimes don't impress me too much.
Anyway we could go back and forth on this quite a lot and I suspect it would be less of a discussion than a shouting match.
But that open source code had certain license parameters attached to it by the people who created the code. So it isn't really a victimless crime.
This is kind of a strange outcome. So, for some, Rybka's alleged plagiarism has introduced a new ethics to chess: Not using an engine that does not have an original code. Even though the engine has proven itself quite capable, and the matter appears to be which manufacturer will benefit monetarily from the product, some people will apparently no longer use the engine because of this. Hmmm...
Is not using illegal software considered strange now?
It's funny to say he stole open source code
I think the point is he used it to make money right? Victimless crimes don't impress me too much.
No, the point isn't that he used it to make money -- Fruit's license (the GPL) actually allows that.
The point is that under the terms of that license he was obligated to also make Rybka open source under the GPL (for a fee, if he wished). If he doesn't want that, then he has zero rights to the code and isn't allowed to use it.
The victims are the people who could have used Rybka's ideas to make their own even stronger engines, just like Rajlich did with Fruit. Why can he do it with Fruit, but we can't with Rybka? He must open the source like the license says.
Now that I know that Rybka had code copied from other engines, I want to look for another free alternative.
I had been using Rybka 2.2, a version that had been posted on the Chess.Com downloads section for free.
Is Stockfish the way to go?
Or Fruit?
I think I'll use Stockfish, but if anyone has any input I'd be glad to hear it.