If you’ve upgraded to 3.1 firmware, you’ll need to wait for the Dev-Team release of Redsn0w or Pwnage Tool for iPhone 3GS firmware 3.1. Don’t try this if you’ve upgraded to 3.1! Redsn0w just makes ...
The Pwnage Tool by the DevTeam hacker group allows you to create custom iPhone firmwares for the purpose of jailbreaking, to get full control over your device (see why you should jailbreak here).