Demoscene Battle Cards
This was done with a group of friends. Each card in the deck is a well-known demoscener, with characteristics such as "coder skill", "huggability" and "arrogance". The aim is (of course) to defeat the other players using your mad skillzzz.
What was particularly great about this collab was the prototyping / testing phase! We basically just used an excel sheet containing all the "cards" to play and balance the game, and found ourselves having a lot of fun, late into the night.
It came second in the wild compo at Revision, and was later nominated for a Meteoriks award - finally, there's a presentation on YouTube