First, shooters in general. Admittedly I'm trying to figure out how to filter out my PC-superiority bias, but keyboard+mouse > controller by a LONG shot for shooters. L4D is a great choice (And on sale for 24 bucks this weekend), Farcry 2 if you like sandbox games, and check out user created mods. Most shooters are on consoles as well, so you probably have an idea of what you may like.
RTS is another Genre where the PC is king. Red Alert 3 is a good choice for the mindless carnage, and Dawn of War 2 should be quite a treat for returning to PC gaming: it has done a lot of new and exciting things.
Its pretty easy to find great games that are slightly old for incredibly cheap. Surf through Steam, check out every weekend deal, and you might find some awesome bargins.
Lastly, if you want unique, innovative indie games, or fun, distracting casual games, PC is where its at. Id recommend And Yet It Moves, Braid (Finally ported to PC), Aquaria, and The Path for really unique experiences. And stuff like Puzzle Kingdoms or Puzzle Quest, Bookworm, Defense Grid: The Awakening, and any number of Popcap or Mumbojumbo games are likely time (wasters) well spent.