Valve is now accepting sign-ups for developers interested in getting their hands on the Steam Controller ahead of its commercial release this fall.
Though the controller is designed to work well even with games that don't specifically support it, this will obviously interest developers who have games on the service and hope to make them work well with what is likely to become a default control option on the service for a large number of users.
If you're curious about the controller, you can read impressions of the final design or watch a video posted by Valve of the controller in action.