Redsnow is a sort of a jailbreaking and unlocking swiss-army knife for older iPhones.
Redsn0w is an easy to use, multi-platform, multi-device jailbreaking and unlocking (iPhone 2G only) tool for theiPhone 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
Please note this will not unlock the iPhone 3G or 3GS. If you have an unlocked iPhone 3G, don’t use this until you’ve obtained an updated version of the YellowSn0w unlock.
For more info, including download locations, check the original announcement.