blizzid on 14 February 2010
BioShock. I also spent a lot of time playing skirmish mode in the Halo Wars demo; sadly, the full game didn't hold my interest for long.
Going back a few years, I spent countless hours playing and replaying the DOOM shareware. I also spend countless hours playing QTest on my college LAN (Quake multiplayer test). Neither of those was exactly a demo, though.
