Mutants & Masterminds - I think Instant Superheroes and Ultimate Power are nice to have, but you could get years of mileage out of the basic rulebook and a good comic collection.
Vampire: The Dark Ages - I ran a summer campaign with just this rulebook in 1998, and had a blast with it. Ironically, I did pick up a lot of the source material used afterwards, but we never really restarted the campaign afterwards.
Dread: A recent entry to be sure, but I think it really works well for cinematic horror.
Call of Cthulhu d20: Since Wizards knew this was a one-shot product, they pulled out all the stops.
Incidentally, why do some of you rate the 5th Edition of BRP CoC above 6th edition? I already have a copy of 5th, and now wonder if I should bother upgrading.
Shawn