NOTE: iPhone 3G S need not apply!
While I don't have anywhere near the midichlorians Jeremy has for all things Jailbreak, I can tell when the infamous iPhone Dev-Team has their game on:
redsn0w is an easy to use, multi-platform, multi-device jailbreaking and unlocking (iPhone 2G only) tool for the iPhone 2G (original iPhone), the iPhone 3G (but not the 3GS) and also the iPod touch (first and second generation). Currently it is available for Windows and Mac OS X (there are some issues using redsn0w with OS X PPC, please use an Intel Mac until we have this problem resolved).
They point out that that redsn0w provides similar functionality to Quickpwn, and provide more instructions in their full post.
As always, experts only and proceed completely at your own risk. If you don't understand something -- put your iPhone down and back away slowly!
If you need help, check out our iPhone Jailbreak Central forums.