Mass Effect 3 saves. In an interview with GamerZines, associate producer Mike Gamble said it "wouldn't be a bad idea" to keep them.
Of course, Gamble didn't go into any specifics -- the save files could affect future DLC or any potential entries in the Mass Effect series beyond the third and final Shepard-focused game. Or maybe it's just so BioWare can forever track those monsters who decided to shoot Wrex in the face. You know who you are and you should be ashamed.