This is a default repo. Now you know, if you want to add new sim to any iPhone, you must unlock your iPhone. So you can do it easily with this repo. Course this repo is very famous for unlock iPhone.
Repo666.ultrasn0w Repo gives you iPhone unlocks utilities to your iPhones. Specially, this repo works on running iOS 3.0 firmware for iPhone 3G. Now ultrasn0w 1.2 is available. It will help you to unlock not only iPhone 3G, but also iPhone 3GS on running iOS 4.2.1.