anything xbox 360 demo :)

easily RE5 demo on the ps3. Ironic that I hate the actual game with a passion.

Either Too Human or Crackdown.

Reasons? Because the amount of time I spent playing those demos was more than I spent playing some full games. Yeah, they were that addictive and, most importantly, fully featured as demos and could be replayed over and over and over trying different things.

Age of Empires 2 Trial(I was small back then, and I played this game for countless hours not realizing that it wasn't the full game)

Battlefield:BC2 Demo(I love sniping!)

Heavy Rain

My fav was the halo 3 beta... Not sure if that counts but its easily my favorite demo.

Actually scratch that i loved the halo CE demo for PC. That game had a crazy community for several years afterwards. It was awesome

I did sink a number of hours to the delay fest called Halo CE's online :P

A great amount of fun when  you had to lead your shots 5 feet infront of a moving target just to hit them.

Killzone 2 & Resonance of Fate

tuff one, but crackdown really sold me on a game i did not think would be any good.

Hmm. Bioshock was pretty good, made me buy the game. Arkham Asylum was also good. Brutal Legend was funny and convinced be to buy a crappy game. But I'll have to go with Fat Princess. I spent dozens of hours playing that demo. If NSMBW hadn't come along I'd probably still be playing it.

As a kid Turrican 2 on the c64.

Modern day, Uncharted 1 demo and God of War 3 demo.