Amahi Home Server - Making Home Networking Simple
Basically built on Fedora 12 (unfortunately), but offers a web interface and one-click installs on a ton of cool shit, like:
H.264 video streaming
Greyhole, which makes the server act like a Drobo (multiple dissimilarly sized drives, using free space for redundancy)
and a bunch of other stuff. Really quite cool, and so far I'm very pleased with it. Best of all, it's free.