Xi3 Corporation is partnered with Valve Corporation on a pint-sized gaming PC that is designed to support Steam and its Big Picture Mode. It's not quite the Steam Box that you've all been waiting for, but that's really only because it isn't actually being called "Steam Box." This is most certainly a box o' Steam.
The press release from Xi3 is unfortunately light on hardware specs, only going as far as saying that it's built "for residential and LAN party computer gaming on larger high-def screens." The grapefruit-sized Xi3 Modular Computer chassis (pictured above) is the company's own design, but the gaming-specific model is described as a "development stage" product.
The modular design fits into Xi3's goal in this "post-PC era," with company CEO Jason Sullivan having stated in the past that "the tri-board design of Xi3 modular computers makes it possible to have computers that can be regularly and easily upgraded and modified." It seems that the company's gaming concept will operate under the same principles, with an eye toward easy, presumably affordable upgrades.
You can learn more about Xi3's hardware on the company's official website and in the video above, though there's not much to be found at the moment about this Box O' Steam if you're looking for specifics. The video above mentions that the "performance" X7A model will be "priced competitively," but Polygon reports that the coming Steam Box -- codenamed Piston -- is merely based on the X7A. That high-end model carries a $999, according to an Xi3 rep.
It's worth noting that there's no mention of operating system in the initial announcement. Xi3 computers run on Linux and it's certainly likely that the Valve-supported Piston will do the same, especially with the launch of Steam for Linux close at hand. Consider this a developing story.