Valve has teased an imminent announcement of an proprietary controller.
The Half-Life and Portal company said today as part of its Steam Machines announcement that, "We have some more to say very soon on the topic of input."
On top of that, the Steam Hardware Beta Test Agreement states that Valve has "developed and produced prototypes of entertainment system hardware and software, including a set-top box running custom software and a game controller (Emphasis added)."
It is not clear if Valve created the controller for testing purposes or if it plans to release this controller to the public. The company's "Universe is Expanding" teaser page features an image of a controller with no additional explanation.
Valve's third announcement this week is scheduled for 1 p.m. EDT on Friday, September 27. The company announced SteamOS on Monday and Steam Machines today.