Everyone with a valid Battle.net account will have the chance to put Diablo III through a self-proclaimed "three-day stress test" starting today at noon, 12:01 pm PTD, until Monday, 10:00 a.m. PDT. The point of the open beta is to ensure as smooth, non-laggy experience as possible when the game finally arrives May 15. You can download the client here.
The open beta will allow players to team up with friends and play all five heroic classes up to level 13. Unlike invited closed beta testers, however, stress test participants won't have access to post in the beta forums or be provided with Beta Bucks to test the currency-based auction house.
Players in Korea (hopefully not "Korean players" in general as the official FAQ states) won't be able to be a part of this beta. To make up for that, they will receive a separate, invitation-only closed beta test where they can play with those in Taiwan, Hong Kong, and Macau.