Back in February some concept art, purportedly from Valve's next new game, leaked in this video uploaded to YouTube. In April, more video surfaced along with a cryptic code name, "SOB." Now some more landscapes have found their way into the wild, and rumor says it as a sprawling space MMO.
"Stars of Barathrum" is its working title, according to this forum thread on FacePunch. The last word in the working title comes from this character in Dota. "SOB" is also its in-house abbreviation, which could also mean Valve wants this stuff out in the wild, to have a little fun with everyone. Who knows.
There are about a dozen more shots in the FacePunch thread, some spacescapes and character concepts, mostly. We've pinged Valve to ask if this stuff is legitimate or if the company has any comment. It's a Saturday and they're on the west coast, so we may not hear back for a while. Any comment made will go here.
[Update] Seeing that these visual assets have been on this artist's personal site for some time—and he is no longer employed by Valve—it's fair to question whether this project is still active. At any rate, their appearance does not constitute a leak—that term implies something coming from a publisher or a development team.
Stars of Barathrum - Openworld Space Game (New Valve IP) [FacePunch. h/t Jens H.]