Today I was hooked on this web browser game and made me realize there should be a thread dedicated to them.
http://agar.io/This one is amazing, it's like the cell section of Spore where you must eat small cells then as you grow start eating each other but multiplayer. It's super popular too, right now there's like 12 thousand people playing just on the US West section. I just play free for all but there is teams mode as well.
Here's a video of someone playing it
FIGHT TO SURVIVE! (Agar.io - Mitosis: The Game)List some other good browser games if you know any.