Windows: Blackra1n continues in the long tradition of jailbreak apps that never say die when faced with Apple updates. It won't unlock an iPhone for carrier switching, but Blackra1n does give you third-party, unofficial apps and features on your iPhone.
If you're already using an unlocked phone, the folks at iClarified haven't been able to test out the jailbreak on an iPhone, but it seemed to work well enough on a first-gen iPod touch. As the developers note, however, if you let the battery run out on your iPod touch or perform a "hard" reboot, you'll have to boot your iPod while connected to iTunes the next time. Hit the link for a download link, and check out iClarified's Blackra1n walkthrough for step-by-step instructions.