Master of Orion 2, when I'd cleared my desk yesterday afternoon but still had 45 minutes left before clocking off. Installed on my works PC are a large number of games, including:
AvP2, Baldur's Gate 1&2, Call of Cthulhu, Civ 4 & Beyond the Sword, Dawn of War & expansions, Diablo 2, Dungeon Keeper 2, Fallout 1,2 & Brotherhood of Steel, Heroes of Might and Magic 5 & expansions, Master of Orion 1&2, Morrowind, Neverwinter Nights, S.T.A.L.K.E.R Shadow of Chernobyl, Sins of a Solar Empire, Starcraft, Summoner and Total Annihilation & expansions.
I think nearly all of those were released before 2006.