Phantasy Star Online - i used to play this game so long i would fall asleep mid-game!
Timesplitters series Virus mode- me and my friend always have to get in some virus mode, and its one of those "one more game" types where you end up playing more than "one more"
Crazy Taxi
Contra
Age of Empires 2- one of those games where it feels like you only played for 30min, turns out its been 3 hours...
puyo puyo series
herzog zwei
Guardian Legend
Castlevania:SotN
simcity
etc.
hell... could go on for a while... but, yeah... just a few of those games that i tend to lose myself in, or just can't get enough of, no matter how many times i played them.