Rpgnow has a new year new game PDF sale. Over 200 PDFs at 15% off and a dozen special bundles with corebooks and supplements for $15 for systems like Scion and Dragon Age.
30% off certain Margaret Weiss books, including the Second dragonlance module, Deep Magic, and a couple novels. Looks like the sale runs through Monday.
I don't remember ever seeing them for sale as PDFs. The stuff I've enjoyed have been the monster books, the celt and norse codexes, and some of the adventures, particularly the big compilation ones.
WotC is having a 2 Year Anniversary sale on D&D PDFs. The Rules Cyclopedia for Basic D&D and the 4e Rules Compendium are $5 each while the 3.5 Rules Compendium is $7 and every other D&D PDF is 15% off. The sale runs through Feb. 22.