Original Diablo playable demo: download
This is the original Diablo game demo which came out in 1997. If you haven't had a chance to experience the original master piece, then I highly recommend giving it a try. It's a classic game which ages well and should definitely belong in any gamer's library. Buy it cheap!
Diablo 2 playable demo: Gamer's Hell File Planet FileShack
The highly successful sequel to the first Diablo game. It improved on virtually everything in the first game and offered an even more addictive play experience. Buy it!
Diablo 3 demo: coming... sometime
Diablo 3 is still in development, and as Blizzard has a habit of only releasing playable demos when the game is near 100% complete, expect for a long wait while the game is finished. Beta testing may be offered, stay tuned for announcements and where to sign up.