We've been watching the development of Firefox 3 for a while, and lately Mozilla has been publishing alpha versions for us to look at that have been at least slightly impressive. While many found the shift from 1.5 to 2.0 to be less than breathtaking, the new 3.x promises to bring a lot of new functionality that makes it a worthy successor.
If you want to take a peek at it, there's a pretty recent review of it available that goes over the latest release. They cover some of the "next gen" features such as unified content handling, as well as some of the overhauled features like malware detection. If the review intrigues you, the Alpha 1 release is still available to take a look at.