Battlefield Heroes is a light weight multiplayer FPS that is free to play and will be funded by microtransactions. It's been in development for quite a while and it's now open for beta testing again. So grab your bazooka and checkout Battlefield Heroes' cartoon combat! Just be aware there may be a wait before you get your beta key.
If you've been following the news, you'll also know that it's main competition at this time is Quake Live which is a browser based multiplayer FPS based off Quake III. Who will survive?