FishEye is a product of Cenqua, makers of (what I think is a) popular code coverage tool for Java, called Clover.
FishEye is a meta-CVS repository. It groups commits into changesets and gives you all sorts of goodies for crawling around your CVS repository. When I first checked it out, it didn't seem like it added /that/ much to the experience, but once RSS feeds on commits and hyperlinks to specific bits in previous revs of the code started making the rounds in the shop I'm currently employed in, it caught on real quick. It's good stuffs.
I've been privvy to an invitation-only beta for a few months now, but now a public beta can be had. It is not an open-source tool, the 1.0 version will cost you some monies, though it does look like you can apply for free licenses if you want to use it with an open-source project.