Demos you admit made you buy the full game.

[Lukeje]

Unreal Tournament. And the full game more than lived up to my expectations.

 

[Notthatbright]

Quake. They gave you the first couple of levels, it was great.

There are probably other demos that I can't remember, but I'm sure they led me to buy a game or two. Mount and Blade was one, actually.

Then the WOW beta, back in 2005? I think? Yeah. That got me hooked.

 

[DarkDain]

Kingdom under fire: circle of doom.
Castle Crashers
Gladiator (old school floppy!)
Dark Stone
Age of Empires
Total Annihilation
Starsiege Tribes

probably more