I generally despise flash games with a burning passion; maybe I've just had a bad experience, but the
vast majority of them are mediocre, get old really fast, and/or are just plain craptacular, which makes looking for the few good ones among them seem not worth it.
I recently came across one I'm uncharacteristically fond of that's a fan-made 2D Flash version of Portal, however, and thought I'd share. Takes a while to load at startup but it has 40 levels, pretty nice graphics, and makes for a spiffy li'l brain-teaser. Only thing that bugs me is it's a bit short for my liking.
[Linky]