Maingear's Steam Machine didn't make it into Valve's brochure. At 4.5 x 4.23 x 2.34 inches and weighing less than a pound, it's pretty easy to miss.
While many of the official third-party Steam Machine partners have been doing their damndest to make a full-size gaming PC as small as possible, manufactures like Maingear (and, judging from the photo, Gigabyte) are taking a different path towards creating an unobtrusive living room box. The plucky little Maingear Spark isn't a gaming PC shrunk down. It's a gaming laptop without a screen, tucked inside a charming little box.