Yeah, you can download the entire thing for free, original game not required. They're replacing all of the textures, models, and sounds that Id Software couldn't release to the public along with the source code.
The lateset version is pretty mature. I've been playing every night for a week, now. There's a handful of decent servers up. They have binaries for all three major OSs along with 64bit support.
-- Roy "Kylratix" Laurie