At midnight, Diablo 3 will be released worldwide. If you're unsure of whether the game's worth your money and you didn't get into the beta, there's another way for you to test out the action RPG. After launch, a Starter Edition of D3 will be available for no charge.
The Starter Edition has more or less the same content as the D3 beta did. Starter players can experience a portion of Act I. This sample culminates with a battle against the Skeleton King. You can test out the multiplayer as well but can only team up with fellow Starter Edition players. Other restrictions include a level cap of 13 and no access to the Real Money Auciton House.
There's another catch with the Starter Edition: you need a Guest Pass in order to get it. Guest Passes will be included in box copies of Diablo 3. It'd be nice if they provided Passes with digital copies of the game as well but Blizzard makes no mention of this.
Assuming you get a Guest Pass, all you need to do is sign into your Battle.net account and download the game client from the Account Management page. You'll need a Battle.net account to do this, obviously, but making one is a relatively painless process.
Should you decide to upgrade to the full version, you'll have to do it through your Battle.net account as well. Presumably you'll be able to carry over the characters you made in the Starter Edition. That's how the World of WarCraft and StarCraft 2 Starter Editions work, anyway.
Before you start sucking up to someone with an available Guest Pass, check out the "What Is Diablo 3?" trailer. It'll give you a nice overview of the game and help you decide whether it's worth checking out.