Way back in November of last year, Blizzard registered a few strange domain names. The first of these to be revealed is projectblackstone.com, which seems to be a bit of marketing for Heart of the Swarm.
To log in to the site, you'll need this password: Y7_$0>0k_3<$m
The password was discovered by a few intrepid explorers on Reddit, who found it within a chunk of encrypted text in the page's source code.
The site itself contains information on Project Blackstone, apparently a secret research arm of Dominion intelligence. There's not a lot on there right now, but I'm assuming more info will be posted over the coming weeks, so keep an eye on it!