After only a couple weeks of release, Amazon's dropped the price on Portal 2 for today's daily deal. The PS3 and Xbox 360 versions are $35, and the PC version is $30. The PS3 version was sold out for a little while this morning, but just popped back up. I assume this sale won't last the whole day, so you'll probably want to get in on it soon. If you somehow didn't cave and grab the game at launch, definitely consider it now.
If punching spines is more your bag, Amazon has also matched store discounts on the brand new Mortal Kombat game, bringing both versions down to $40 for the standard editions. While this isn't a daily deal, it's not clear how long the sale will last, so you probably don't want to sit on this one too long either. After years of missteps, this one seems like it's gotten people excited about the series again.