iPhone/iPod touch only: If you've been waiting to upgrade your iPhone or touch to the recently released iPhone 2.2 software until you could jailbreak the update and run your must-have jailbreak apps, you're in luck: The iPhone Dev team has already released QuickPwn for both Windows and Mac and the PwnageTool for Mac. Not only will jailbreaking your phone with one of these simple-to-use apps add both Cydia and Installer to your home screen, but it'll also give you access to a few other very cool iPhone 2.2-only hacks.
Enable Google Maps Street View on the iPod touch
For example, if you were one of the many who were upset that the iPod touch update didn't include Google Maps Street View, you can enable Street View on your iPod touch with a few simple edits. (NOTE: Currently the jailbreak only works on first generation iPod touches, so if you've got a second gen touch, you're going to have to wait.)
Enable Emoji Icons
Similarly, if you were disappointed that Emoji icons weren't working on your iPhone after the update, this other simple hack enables a ton of Emoji icons on your iPhone.
Beyond that, jailbreaking means you've got access to the many cool iPhone apps for jailbroken phones that we've covered in the past.