This method does upgrade your baseband, which means it will negatively affect unlocked phones. You should read below for details.
Spirit is comparable to redsn0w.
It does not require an IPSW file, but thereby is specific to one exploit: iPhone OS 3.1.* / 3.2 (all devices).
Steps: Download Spirit. This will come in the form of a single .exe file (Windows) or .zip file (Mac OS X).
Use iTunes to upgrade to the latest firmware. Spirit runs on already upgraded un-jailbroken iPhones.