Amazon just announced an agreement that makes Prime Instant Video the only place you can watch HBO originals online without an HBO subscription. Starting May 21st, Prime members will get exclusive, unlimited streaming access to pretty much every HBO original you're interested in except Game of Thrones.
Right out of the gate, Prime members will get access to every episode of The Sopranos, The Wire, Deadwood, Six Feet Under, Rome, Eastbown & Down, Enlightened, and Flight of the Conchords; and select seasons of Boardwalk Empire, True Blood, and Treme. Early seasons of Girls, The Newsroom, and Veep will be included as they pass the three-year mark from original airing. Sorry True Detective fans, this agreement doesn't include Rust Cohle.
Technically, this marks the first time HBO's original content has been available for non-HBO subscribers (not counting HBO Go users freeloading on someone else's subscription). And right now, Prime Instant Video is only available to US customers, meaning this agreement probably won't reach beyond US borders for now.