redsn0w

A cross-platform jailbreaking, unlocking, and customizing tool for iPhones and iPod touches. Customizations include boot logos, recovery logos, and "verbose" boot.

It's a standalone program that doesn't use iTunes (no custom IPSWs are involved).
Apple Firmware 4.1-5.1.1 supported. Hardware supported. iPhone 3GS iPhone 4 iPhone 4S iPod Touch iPad iPad 2 iPad (3rd generation)