George Hotz (aka Geohot) who had released a software called purplera1n to jailbreak iPhone 3GS only for Windows users has now released purplera1n for Mac users.
He has also released updated version of purplera1n RC2a for Windows users which includes some more payload stability improvements.
version was updated to RC2a. There is no reason to run RC2a if you
already have RC2 installed."
You can download purplera1n for Mac from this link.
Once you have jailbroken your iPhone 3GS using purplera1n, you can also unlock it using UltraSn0w. You can checkout our step-by-step guide to unlock iPhone 3GS using UltraSn0w.
Please don't forget to take a backup of your iPhone (refer to this post for more details). Please proceed with caution as jailbreaking your iPhone could void it's warranty.
As always, please let us know how it goes in the comments section below.[via Geohot's blog]