While Nidhogg does feature online play, Share Play allows you to play with a friend that doesn't own it!
Outlast can be a terrifying experience but it quickly becomes a funny one when you are watching your friend scream.
Towerfall is one of the best multiplayer experiences in gaming so getting to play it online is awesome.
While JS Joust isn't playable in Share Play the other games in this fantastic compilation are!
Like most of the games on this list, Starwhal is a fun local multiplayer experience that lacks an online mode.