It's gone through a few iterations, but the Steam Controller is finally ready for prime time and will be out this November. To coincide with the multitude of Steam Machines being released, you'll be able to pick up a controller or an extra one if you so desire.
The Steam Controller will retail for $49.99, it's got dual trackpads, a thumbstick, four face buttons, and triggers. Built in are also haptics for vibrations and it connects to your computer wirelessly.
I'm cautiously optimistic the trackpads will work well, and I can see them only being used for certain types of games. This should be interesting to see how well the controller fares after being delayed and going through multiple redesigns.