Listen up. From noon on Friday, April 20 (Pacific) until 10:00am (Pacific) on Monday, April 23, Diablo III will be in open beta. Meaning anyone with a valid Battle.net account can get in on the action.
To download the client now ahead of time, click here. Once downloaded and once the weekend rolls around, you'll be able to play all five classes up to level 13.
If you don't have a Battle.net account from WoW or StarCraft II, it's free. You can get that process started here.
Note: the beta is worldwide, the times above are just for reference. So if you're in, say, Australia, it'll run from 5am on Saturday until 3am on Tuesday, April 24.