A Chess.com bot.
NOTE: This project will no longer be updated (this project became quite popular and I had to take it down after the website asked me--don't condone cheating!).
Download stockfish and put it in the project's root directory (check out the Config.java file and make sure the executable names match as well).
Make sure you're using Chess.com's "old" interface and on the simple board in Live Chess with classic theme/default 68x68 tiles and algebraic notation is on.
That's pretty much it. Hit play and, well, enjoy.
BTW, I cannot stress enough that your board size is 68x68 and looks like this (otherwise you have to change board width/tile size in the config class).
If there are any bugs feel free to create a new issue. The only current feature not implemented is pawn advancement.