purplera1n

Thanks to the combined efforts of AriX and westbaer, they are releasing a version of GeoHot's purplera1n jailbreak for the 3GS for the Mac. Both Windows and Mac versions have also received payload stability improvements, updating the the Windows version from RC2 to RC2a (although there's no reason to use it if you already have RC2 installed). You can get both the Mac version and the new Windows version at purplera1n.com.

In response to the Dev Team's decision to wait until OS 3.1 to release a jailbreak for the iPhone 3GS, George Hotz, aka Geohot, has released his own jailbreaking tool for the iPhone 3GS called purplera1n, as he announced on his site. Right now it's only for Windows, but he says a Mac version is next on his list. It's still in beta, and many of the comments on his website are from people experiencing issues like frozen iPhones and compatibility problems with Vista, but an equal number of comments say it works just fine.