Blizzard has announced that the Diablo III starter edition is now available and free;
The demonically-besieged world of Sanctuary needs heroes. Now you can join in the apocalyptic battle for FREE via the all-new Diablo® III Starter Edition. Available exclusively via Battle.net®, the Starter Edition allows you to fight your way up to the Skeleton King boss in Act I, and advance all the way to level 13, without having to purchase a copy of Diablo III.
Seems like a reasonable restriction to get a taste of the full game but from memory it's the first time Blizzard has introduced a "free" sort of game without time limitation and instead relying on the games enjoyment to make you become a purchaser.