I was checking out some stuff on Paizo's website when I noticed that they were offering some Mutants & Masterminds products for up to 50% off. The prices aren't quite as great as the RPGNow sale that ended recently, but there are still some damn good deals to be had.
My personal favorite is the Pocket Player's guide for $11.97. I'm very particular about keeping my books in good condition and unfortunately most of the people I have ever gamed with do not share this view. It seems especially true when they are handling my books. That is why I keep one of these on hand, so my main rule book doesn't get ruined. I only wish more publishers offered these cheaper, yet very useful pocket guides.
My other personal recommendations are the Golden Age sourcebook, The Book of Magic and Ultimate Power pocket guide. All of these are under $20 and the Ultimate Power pocket guide is as great to have around as the Pocket Player's guide.
The Green Ronin Mutants&Masterminds sale ends February 18, 2010 so check it out while the deals last.