You've probably already seen the Steam Machine that third-party manufacturer iBuyPower is planning on releasing later this year (pictured above). While Valve has made its own prototype, the official Steam Machine roll-out will be dependent on the efforts of Valve's third-party partners.
Although the company will have a press conference at a pre-CES event later today, a few partners have already been leaked early.
According to Engadget, there are at least twelve third-party partners participating in the Steam Machine program: Alienware, Falcon Northwest, iBuyPower, CyberPowerPC, Origin PC, Gigabyte, Materiel.net, Webhallen, Alternate, Next, Zotac and Scan Computers. The site says that "the entire lineup" will be available at CES, which officially begins tomorrow. Of course, it's likely that more details will drop at Valve's conference--set to begin around 5PM PT. Shacknews will be covering, of course.