The iPhone Dev-Team have pushed a small update to the Mac OS X version of RedSn0w that brings support for OS X 10.5.x and below.
This update brings the Mac OS X version of RedSn0w t0 version 0.9.10b6b, and if you have an older Mac computer then this will come as great news and allow you to Jailbreak your iPhone, iPod Touch or iPad running up to the iOS 5.1 firmware (tethered of course).
Update #1b: The OS X version of redsn0w has been updated to fix an issue for those running OS X 10.5.x or earlier.
Download RedSn0w 0.9.10b6b for Mac OS X
You can find instructions on how to jailbreak your iOS device here.