Gish isn't a Mac-only experience, but as a platform game, it's a pretty good one. I played it way back when it was released on Windows, but Inside Mac Games reports that Chronic Logic has released a universal binary of Gish, so now you can play it on Intel or PowerPC Macs alike.
Like I said, I found the game pretty fun. You play a ball of tar (no kidding) that rolls through underground locations in search of his lady friend (isn't that always how it goes), and the fluidity of your main character brings a lot of weird physics manipulation into play-- you can squeeze through small places, and even stick to surfaces like walls and ceilings. It's definitely a fun twist on platforming (and Gish can even invade old platformers with "warp zones," little psuedo stages that let you visit other games with Gish's abilities), and if you're in the mood for a great indie game, make sure to check out the demo. The full game is $20 over at the Chronic Logic website.