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Most chess sites provide a "firehose" for current games. It should be pretty easy to pipe a game's pgn file to stockfish.

Also, sites also have a "viewer" page for most games where they add a chess engine evaluation of the current position. All you really need to do to cheat is to open your game under another account and you can just copy over the move in the top line.

If you are interested in super computer evaluation of chess games, [sesse](https://analysis.sesse.net) usually has the analysis of the "game of the day". It's run by people at chess24, so they usually analyze Magnus' games.

Stockfish is free and open source.

https://stockfishchess.org/

If you want to just goof around, use any chess program (such as OSX's built-in Chess.app) set at the hardest difficulty, and play the opposite color from your real game. Then copy your opponent's moves to Chess.app, and make the computer's moves in the real game.