I don't know if this has been cover before, but I'm always looking for new fun browser based games... I'll start with a few of mine, but I'd like to hear from ya'll.
Full Moon: This is a short, fun game that got me thinking. This game designer has a lot of fun brain twisting games, so if you like this one, check his others.
Icebreakers: Save the Vikings! Slice ice with your mouse. There's a few newer ones, but this is the original.
Don't Look Back: A retro style jump/shoot game based on the Greek Orpheus myth.
Closure: I just found this one, still working on it, not sure I can explain it...
Fantastic Contraption: A physics/machine-building game. Now that I think about it, I think I found out about this one from this forum a while ago.