If you were hoping to get your hands on an early preview of Diablo III but never got invited to the closed beta, Blizzard has a pretty sweet weekend freebie. Right now, you can participate in the open Diablo III beta absolutely free! Until Monday at 10am PDT, you'll be able to play as each of the five classes up until level 13 but that should give you plenty of time to test it out and see if your computer can run the game smoothly and if you should put down your hard earned cash for a pre-order.
Getting started is easy and you'll want to follow the steps here.
The download is pretty large so you'll want to make sure you can afford the bandwidth before beginning.