Can I direct your attention to a little something I've been enjoying lately? And I'll warn you in advance that being free, flash-driven and browser-based it strays dangerously close to the dark provinces of the casual game.
It's Amorphous +; I'm playing the thing through Kongregate but you can probably find it on any number of game portals.
Basically, you're a little dude with a sword so ridiculously large it puts the cast of Final Fantasy to shame. Your job is to swing the sword, and splat jelly-like blobs before they splat you. It sounds simple enough, but the real skill is in earning combos by splatting multiple blobs in a single swing. That gets complex as the different blobs have interesting interactions - Glooples can merge to become a significantly more dangerous larger species, Inkies leave blinding ink that Toasties can set on fire, and Melties leave a pool of acid on death which kills anything that crosses it.
All these shenannigans should be run of the mill but somehow they're fantastically entertaining. Plus there's a whole mess of awards to get for doing any number of crazy things, which makes the whole thing start to get deeply addictive.
There's a good couple of hours of gameplay in this thing. And it's free. I highly recommend it. Go play it now.