It may look a bit rough and the parts don't quite fit together, but Machinima.com's recent Half-Life 2/Castlevania mashup is pure genius aimed squarely at anyone who played both the NES action game and the more recent PC shooter.
As the name suggests, the map is a remake of the first level of the NES version of Castlevania within the Half-Life 2 engine. The map's creator didn't go to the trouble to recreate the former's weapons or enemies, but somehow the video is that much more appealing for his laziness.
Maybe I'm just a sucker for that classic theme song, but headcrabs and those damn shrieking zombies from Ravenholm just seem to fit in with Konami's gothic setting, even when its obvious 8-bit heritage is almost being subconsciously mocked by the transition to (relatively) high resolution 3-D.
Nothing official will ever come of this map, and I wouldn't even expect its creator to have the attention span to recreate the entirety of the NES game, but like peanut butter and basically anything, Castlevania and Half-Life 2 is yet another flavor combination that works surprisingly well together