I don't know many games that work well for two players, so I'll just list some multiplayer favourites of mine. Starcraft is a must of course. Half-Life and it's various mods (Team Fortress Classic, DOD, CS, Natural Selection, Science and Industry, Firearms, and others). Half-Life 2 is another good choice for multiplayer mods, as well as DOD: Source. Both HL and HL2 will keep you busy for years with the number of quality free mods you can get. Battlefield 2 is also a good choice.