Limera1n has been released yesterday evening, and after geohot got angry and pulled the download link (I don't know exactly why), it seems that Limera1n is now available for download again on the official website. This new jailbreak tool promises to jailbreak almost any device running iOS 4.0 or 4.1.
The supported products are iPhone 3Gs/4, iPad, and iPod Touch 3G/4G. For the unmentioned products: iPod Touch 2G support is coming soon, while for the iPhone 3G you must use redsn0w. For the moment, limera1n is available only for Windows, the compatibility for Mac is coming soon.
Theoretically, limera1n does hacktivate your device (allowing you to use it with any carrier), but many people reported problems after hacktivating their iPhones with limera1n, so I don't recommend you doing this yet (limera1n is currently in beta 4).
Hope I don't have to tell you that you're using it on your own responsibility, it is an unstable release so don't expect from it to be bug-free. You might have to restore your device or you may even get a bricked one as a result of using limera1n!
Download from here.