Spelunky is not only my indie game of the year but probably my favorite game of the past couple years, indie or otherwise. Info below cribbed from the September thread.
Spelunky - $7.49 (50% off)
Edit: I guess I should add I know this isn't necessarily an "overlooked" title but last time it was on sale, I saw people hating on it in the Steam thread. The game is excellent and people can say they don't like it but you cannot argue with the depth of gameplay and excellent design. It seems like most games that are "hard" today get labelled as bad, don't be fooled into believing that is true for Spelunky! To put into perspective how excellently made the game is, I have found only ONE thing I would change about the game if given the chance. At first glance, I have thought many times that something should be changed only to realize how brilliantly crafted all the pieces already are and doesn't need changed. Most games I can find several things that I would like changed but put up with them because I like the game otherwise, Spelunky is the exceedingly rare exception.