Some of the better flash games I've played in the last couple months:
Realm of the Mad God -- a stripped-down MMO roguelike where you pretty much just shoot monsters and collect loot. Unlock player classes as you play. Free except you can spend real money to buy perks (I never have, and never will.)
Soul Brother -- a nice puzzle-platformer where your ghost can inhabit different bodies (each with different capabilities) to get past obstacles.
Lesbian Spider Queens of Mars -- a pitch-perfect retro arcade game. Also has a sort of Flesh Gordon/Barbarella vibe.
Upbot Goes Up -- a solid puzzle game with some challenging levels. I have beaten them all except #31.