The Elder Scrolls 5: Skyrim DLC, Dawnguard, will be going into beta for it's released to the public. The PC beta for Dawnguard was so successful, Bethesda has decided to offer a beta for the Xbox 360 too.
If you want to apply for the Dawnguard Xbox 360 beta, sign up! Applicants will be selected in a week, probably after E3. The beta is the full version of the DLC, complete with achievements so if you're chosen, you won't have to buy it. You'll have to own Skyrim though.