Taht's Realm Crafter
http://www.realmcrafter.com/catalog/information.php?info_id=6According the news, they're leaving Blitz3D using ther own C++/DX9 engine currently.
There are two versions of Realmcrafter currently in development. The older BlitzPlus/Blitz3d based version is now called Realmcrafter Standard and the newer C++/irrlict version is called Realmcrafter Pro. The developers have said that they will continue to support the standard version, but their main efforts will be focussed on the pro version.
I believe the standard version comes with source code for the client, server, and game editor.