On Blizzard's battle.net site, they have announced the release of demo versions of the highly popular game, Diablo III, for both Microsoft's Xbox 360 and Sony's PlayStation 3. The demos allow you to play up to the Skeleton King in Act I, which isn't very far.
There is also a "Starter Edition" for both PCs and Macs that allow for those users to experience the game world too. In all fairness getting that far into the game will give you a good idea of what the game mechanics, gameplay and story is going to be like if you purchase the full version.
Diablo III is an online only game (which caused quite the ruckus when it was first revealed) so you will need a battle.net account in order to play either the demo or the full game.