Just about every game I try to buy from the bargain bin these days. Perhaps it's my extended period of living at a college student's income, or perhaps it's just that now that I'm a little older and wiser I'm not so prone to buy for $50 now what I can get for $30 or less in a few months with a little patience.
That, and just about any console game can be got from GameFly as part of a flat monthly rental rate and with often a formidable discount if you decide to keep the copy. The only games I really need to buy these days are for the PC.
In terms of games I got out of the bargain bin lately, I picked up Mass Effect for the PC for $20. Space Siege was for something like $7.99 at a local Target.
If Indy games which were $20 to start with count, I haven't regretted dropping $20 on Defense Grid.
I also use GameTap, which is pretty much the bargain bin digitally piped to you, with not a few still well-priced games (e.g. Gothic 3, Puzzle Quest).
I'm currently waiting for Dead Space to come down in price to about the $20 mark. Direct2Drive has brought it down $10 to $39.95 already. Maybe the only game I'll buy full price any time soon is Champions Online, and that's only because there's an inherent advantage to playing a MMORPG on release day.