purplera1n is comparable to redsn0w.
It does not require an IPSW file, but thereby is specific to one exploit: iPhone OS 3.0 on the iPhone 3G[S].
Steps: Download purplera1n for Mac OS or Windows. This will come in the form of a (compressed) .zip file (Mac OS) or a single .exe file (Windows).
Use iTunes to upgrade to the latest firmware. purplera1n runs on already upgraded un-jailbroken iPhones.