Joystiq reader Ryan stumbled upon a multi-player browser-based first-person shooter (that's a mouthful) called Phosphor. The game, developed by Rasterwerks, is in beta so it's not without it's problems. It locked up our mouse in Firefox when playing, but the overall initial experience was rather smooth and impressive. Check it out!
Note: this baby takes more load time than a PSP and you'll need to have Shockwave 10.1 to run it.