Are there any good "best move" generators online or free to download?

I like to study chess in my spare time - I don't play it with others much, just study moves, play against myself, study famous games and analyze the pros and cons of different moves.



I've been looking for a good program where you can input moves up to a certain point, and then have the computer/program generate what it thinks the next "best move" (relative, I know) would be; allowing you to study why it is the best move or what possible strategy the computer might be taking or assuming you might take. If it has tips or an explanation as to why the computer decided on that move it would be fantastic, but I know that's a lot to ask from a free program.

If this type of program doesn't exist, maybe a chess game where you can change the computer's moves or manually input them, and then continue to play against it? Something of the sort.



It'd also be great to input an old game you lost and figure out where you went wrong, or what you could have done to win :DAre there any good "best move" generators online or free to download?
Um, using a computer to play your moves is cheating. Using a computer to play your opponent's move to see if tyour brilliancy misses the attack on your queen.



Anyway, these are free.



http://www.tim-mann.org/xboard.html



Any chess program does that.