The download links for redsn0w are:
Mac OSX (x86) 
For now, the redsn0w beta release supports only the iPhone3G and iPod Touch 2G at today’s 4.0. It’s still a beta, so you’ll need to let Cydia reorganize, reload, and update after using redsn0w.
The Dev Team also had following updated regarding the same post;
Update #2: There’s a new redsn0w beta (links were changed above) that should fix any iBooks problems people were seeing. Just run this new version 0.9.5b5-4 and deselect Cydia (you don’t want to reinstall Cydia over itself).
Update #3: Remember, there are scammers everywhere in the iPhone scene. The latest one involves something called “ClawPack”. Avoid this costly, untested, and certainly unendorsed ripoff of our free software.
Update #4: There’s a new redsn0w beta (links were changed above) that should fix any APN or MMS issues that users were seeing. It’s safe to re-run it on an already jailbroken iPhone without restoring…just deselect “Install Cydia” if you do that.