Half-Life 2's best single-player mod (though Mistake of Pythagoras is a close second) was created by a single person, one Adam Foster. It impressed Valve enough for them to put it on Steam. Minerva sees you fight your way around an island and into a Combine-occupied WWII bunker.
Above: For a Combine-infested hell-hole, Minerva's island is surprisingly cheerful. Oh, hold on ...