http://www.rasterwerks.com/This game is called "Phosphor," by Rasterwerks. It's a flash FPS (first person shooter), but you can also connect and play online; web-based, no downloads needed. (Although you have to install a few things.) If you're using Firefox (like I do), Phosphor doesn't play well under Firefox, so open up IE or AOL. (I suggest IE) Anyway, enjoy Phosphor!
Sadly, it's online community is dead. No one is ever online. It has offline play, but it gets boring...
It has background music, it's 3D, and overall... IT'S FREE.
If you ask me, it's an equivalent of Unreal Tournament. Only it's
free.