In a move that echoes their approach in StarCraft 2, Blizzard has announced today that Diablo 3 will get a free demo that they're calling Diablo 3 Starter Edition.
In the first 30 days after release it will work similarly to how StarCraft 2 worked when it released in 2010. Boxed copies of the game will come with a Guest Pass that enables a friend to play for free. However, free players will only have access to Act 1, and a level cap of 13 (the same content as the beta.) They will also not be able to access the auction houses.
After the first 30 days it will be available as a download from Battle.net.
Blizzard has also announced that Diablo 3 will get online support in the form of character profiles on Battle.net. In these profiles you'll be able to take in-depth looks at all of your characters, their skills, and their equipment (including all dyes, gems and whathaveyou.) You'll also be able to get a glimpse at other players' characters if you're looking for tips on how to maximize your build.