This is geohot's jailbreak utility. It is an updated version of purplera1n but now uses geohot's implementation of the usb_control_msg(0x21, 2) Exploit. It has been released for Windows and Mac. It jailbreaks all devices on 3.1, 3.1.1 and 3.1.2. It even jailbreaks OTB iPhone 3GS and iPod touch (3rd generation), however tethered
As of 3 November, blackra1n also unlocks and hacktivates the iPhone 3G and 3GS running baseband 05.11.07 (pushed out with the 3.1 release). After installation, opening the 'Blackra1n' app on your iPhone will reveal a blacksn0w option for unlocking your iPhone.
All you have to do is head to the blackra1n website and download the application on to your computer. Then, plug in your iDevice and press "Make it ra1n". Then you wait. As this is a tethered jailbreak, the iDevice may not boot. If is does not boot, just press "make it ra1n" again. This should only happen with newer iDevices, though. These are the updated (after week 40, 2009) iPod touch (2nd generation) (MC Model), iPhone 3GS and the iPod touch (3rd generation). These devices have updated bootroms that are not vulnerable to the 0x24000 Segment Overflow.
Installing Package Managers
The Blackra1n procedure installs an App on to the device, aptly named "Blackra1n" that handles the package manager installation. With it you can install Cydia or Rock. After installation, the device will respring and the package manager of your desire will be on your SpringBoard. After installing, you may remove the Blackra1n App using the "uninstall" button within it. Don't worry - this does not remove the Jailbreak, just the App to install Package Managers if you no longer need it
Integration with Blacksn0w Unlock
On the iPhone 3G and iPhone 3GS, there is an additional section called 'Sn0w'. If you install this, it will unlock your 05.11.07 baseband after which it will restart your CommCenter - no reboot required. This unlock is detailed on the Blacksn0w page. If you need an unlock on the iPhone, use BootNeuter as it will always work with every firmware.