Amnesia: The Dark Descent and the Alan Wake franchise. RPGs have taken a critical price hit as well, including the Witcher franchise, Fallout: New Vegas and Krater.
Still not satisfied? Dip into even more savings on Plants vs. Zombies, Sniper Elite V2, Gratuitous Tank Battles or yet another Indie Bundle.
Update: If you're looking for Sniper Elite V2, Amazon actually has a better deal and is currently offering a PC download for $15.