Below you can find links to download PwnageTool, a utility released by the iPhone Dev-Team to build custom IPSWs that jailbreak the iPhone, iPad, and iPod touch.
PwnageTool 5.1.1 (Mac): PwnageTool_5.1.1.dmg
PwnageTool 5.0.1 (Mac): PwnageTool_5.0.1.dmg
PwnageTool 4.3.3 (Mac): PwnageTool_4.3.3.dmg
*We will be updating this list with new versions and mirrored download links as necessary.