Demo that made you want to buy a game?

Call of Duty, did both the demos and was impressed, then I went off to buy GTA:SA but found out they didn't have the game there, but I did saw CoD and the expensionpack UO, was a great deal.

Also, FEAR.

Sytzepunk

Same here.  I had NO interest in getting a WW2 shooter, but after playing the 360 demo I drove to the store and bought it.  Of course, as I posted about a year ago it has worked in the opposite direction many more times -- where I will want a game and then not get it after having played the demo. 

Fight night round 3 demo made be rent the game (twice) and eventually buying it after the price was already $29.99.

Saints row demo made me buy the game without renting it first and I ended up regreting it and sold it to someone.

 Recently I've played the demo for DiRT many times and am now waiting for it to come out so i can buy it.

Worms, Wipeout, Tekken 3 and... Superman Returns (thank God reviews came out soon after that).

Ratchet and Clank, Oddworld: Abe's Odysee and the original Metal Gear Solid.