Play local co-op without the couch.
Steam Remote Play Together

Valve just launched their first public beta for the Steam Remote Play Together feature. This will allow you to invite online Steam Friends to join in on your local co-op game, local multiplayer, and split screen games as if they were sitting beside you.

You can have up to four players (or more in some situations) instantly join on your game. For this feature, only the host needs to own and install the game. The other players will connect via the Steam Remote Play streaming technology.

Valve put up an announcement detailing how this feature works. To begin, you will need to be enrolled in the current Steam Beta.

From there, simply launch a local multiplayer game and invite your friends using the "Remote Play Together" invite option. Once they join, their controllers will act as if they're plugged directly into your computer. Your desktop will never be displayed. In addition, you can share or limit access to your keyboard and mouse.

Obviously, this won't be a problem when playing with your friends, right? You can trust your friends... right? They would never do anything bad... riiiiight?

Some of the supported games can be found here in the Steam Store.