In all seriousness, there's so much stuff out there, you might do better by narrowing your search a bit. What sort of things do you look for in a sourcebook? Are you looking for a book of monsters, or a book of spells, feats, and PRCs? Do you want a Campaign setting or something else with lots of fluff? or do you prefer Crunchy bits? Does it have to be Wizards of the Coast, or would you prefer a 3rd party D&D supplement? Or do you want something less D&D specific like Mutants and Masterminds, Arcana Evolved, or D20 Modern?
Or hey, even better: you could give your gift certificate to me. I won't open my presents until tomorrow morning, and I just can't wait that long.