Stockfish is available in the App store for free.
I’ve used it for a few minutes. Here is what I know:
It’s not nearly as good as the iPhone/iPad version. But if they can make it do exactly what the iPhone app can do, it will be incredibly useful. And I’m sure they will reach iPhone/iPad level, and I wouldn’t be surprised if they have more useful features.
Right now you can enter in moves and use infinite analysis. This is way too advanced for most people.
You can’t play against it (honestly, no one good at chess needs a computer to play against).
It’s great for entering your notated games and emailing the moves to to someone.
You can open up PGN files with it. This isn’t incredibly user friendly, and recommended only if someone sends you a PGN file. I like the iPhone and iPad app because one can email you a PGN file (see below) and it automatically imports from your clipboard.
Or you can copy and paste the following, and save it as a .pgn file (use TextEdit on Mac or Notepad for Windows).
[Event "USSR vs. Rest of the World"] [Site "Belgrade SRB"] [Date "1970.03.31"] [EventDate "1970.03.29"] [Round "2.1"] [Result "0-1"] [White "Bent Larsen"] [Black "Boris Spassky"] [ECO "A01"] [WhiteElo "?"] [BlackElo "?"] [PlyCount "34"] 1. b3 e5 2. Bb2 Nc6 3. c4 Nf6 4. Nf3 e4 5. Nd4 Bc5 6. Nxc6 dxc6 7. e3 Bf5 8. Qc2 Qe7 9. Be2 O-O-O 10. f4 Ng4 11. g3 h5 12. h3 h4 13. hxg4 hxg3 14. Rg1 Rh1 15. Rxh1 g2 16. Rf1 Qh4+ 17. Kd1 gxf1=Q+ 0-1