Double Fine is having a 50% off sale, effective till the end of 10/15.
I'm only aware of it being on XBLA, but it might be on other platforms as well.
I just picked up Iron Brigade (and am loving it so far) for $5. I also nabbed the DLC for Stacking for 200msp (about $2.50)
Anyhoo, the following are on sale for today and tomorrow...
Costume Quest - reminded me a lot of Mario RPG, and is the best damn Halloween game ever created
Costume Quest: Launch TrailerIron Brigade - a WWII alternate history
tower defense game, wherein the good guys (you) have mech like vehicles which can be heavily customized (the players character can be customized too). You can also lay stationary defense turrets of different types and power. I played the demo, not being particularly interested but thinking I'd just give it a shot seeing as it's a Double Fine game, by the time I got to the end of the first level though I was sold (especially for only $5). As with other DF games, I find myself laughing quite often while playing.
Iron Brigade TrailerStacking - a puzzle game where you....you know what. Just go watch the trailer. Also a laugh riot.
Stacking PC Trailer [HD]All three games have DLC available as well, also %50 off.