Teenage hacker Geroge Hotz, who was first unlocked the iPhone, released Purplera1n yesterday in a blog post titled "I make it ra1n" that unlocks the new iPhone 3GS.
Purplera1n is available for Windows only, a Mac version will be available shortly. Purpera1n runs on iTunes, and installs an app called Freeze.
Freeze then installs the installer Cydia, which lets the user download applications not authorized by the iPhone app store.