Right now, instead of reading this, you could be playing Leap Day. If you have the Unity plug-in in your browser and your teacher/professor/boss isn't looking, go do it.
Or at least load this up (and then scroll to the amazingness lurking in the upper right of your screen. What you'll see there is a saved game from one of the game's developers who has already played Leap Day. His game went into public beta yesterday.
Leap Day is a simple, browser-based co-op strategy game that has you laying down roads and houses in order to get a bunch of cute little leaping worker guys to make better and better materials. Oh, that sounds so tedious. But it's not! It's like Sim City but just with glorified conveyor belts. It's like SpaceChem but cuter!