A recent update to Steam comes with a beta release of Remote Play Together.
Put simply, Remote Play Together allows you to play games online with your friends. That is, games that feature local (split screen or "couch co-op") multiplayer only, with no networking capabilities, can now be played online with your friends. And only the host of the game needs to own the game in their library.
Or put another way, if a game supports multiple people playing on the same device simultaneously (usually using gamepads) then it can now work online through Remote Play Together.
Anyone up for some gaming? I have a number of local co-op games that I haven't really played much because I never really had anyone to play with. It might be fun to organize some play sessions.