Dark Void PC.

I played something like 7-8 hours with Burnout Paradise demo.
But I think I played much more with Ascendancy, an old strategy game for PC. The game was intruguing, plus I found a way to hack the savegames, so I could play beyond the time limit (it had other limits, anyway, so I bought the full game later).

Uncharted 2 MP demo. way too much.

Panzer Dragoon for the Saturn - more than 100 hours. It couldn't afford a game back then so the demo disk on the Saturn was the only thing I could play for more than half a year. Panzer Dragoon sticks out as the best of the bunch and help me purchase the game at a later date.

Current gen: GT5 Time trail, spent a lot of time with friends trying to beat each other time, I suck at this and almost always come last. Didn't play it by myself but 20-30hrs for sure.

BFBC- around 7 hours.

Battlefield:Bad Company 2 Demo. 10+ Hours.

resident evil 5 12+ hours

When I was a kid I played the Sonic CD (For PC), Gex and Heroes I and II demoes over and over and over.

FIFA 10 hahaha I played it every single day, about 5 o 6 matches a day. Until the game came out

I played Sonic Adventure 2: The Trial about 80 times.