The Steam Controller is a key part to their living room and console experience. This controller, among other features, has dual trackpacds, has a new generation of super-precise haptic feedback support, a touch screen, and multiple buttons. The Steam Controller also has a legacy mode where it just acts as a keyboard and mouse for games not optimized for this controller. The beta units are wired USB-based while the retail units will be wireless. The input device announcement was one of the announcements I anticipated for today. The retail version of the Steam Controller is expected for debut in 2014 alongside the Steam Machines.
Steam also says the Steam Controller was "designed from the ground up to be hackable. Just as the Steam Community and Workshop contributors currently deliver tremendous value via additions to software products on Steam, we believe that they will meaningfully contribute to the design of the Steam Controller. We plan to make tools available that will enable users to participate in all aspects of the experience, from industrial design to electrical engineering. We can’t wait to see what you come up with."
The Steam Controller will work with PCs too and just not Steam Machines. Given the Linux-based SteamOS, Linux support for this gamepad will surely come, but no details have emerged yet. It's most likely to be implemented via an open-source HID Linux driver.
More details on the controller from Valve have been shared on this SteamPowered.com page.
Next week Valve also says they will be releasing details concerning their SteamOS prototype.