I am involved in the development of some open source software.
(I will tidy this up later, but listing things briefly so I don’t forget for now.)
Most significant is Conjure.
I contributed (a small bit, but I’d love to do more, not enough time!) to the compiler of the awesome dependently typed programming language Idris.
I contribute to Savile Row – admittedly more often indirectly via making Peter Nightingale do a lot of (very cool!) work.