$4.99 $2.99 until 04/05/2009: Sentinel
Sentinel is a kickass tower defense game that normally goes for $4.99. Chris J. reviewed it about a month ago and loved it, and for the next few days you can get it for a couple bucks less!
Also on sale (FREE!) is Sounds real, a collection of high quality sound effects for your podcast or whatever else you’re working on.