Last weekend I wrote a new library, stb_connected_components.h, which solves a problem a friend of mine was having, but which is also generally useful.
I wrote the initial version entirely on a 5.5 hour twitch stream. This includes about 45 minutes creating some test data and a reference solution, and some time at the end optimizing things.
There were additional updates to the library since the initial version, one to reduce memory by a factor of 10 in the 1024x1024 case, and some which fix some degenerate cases to have better performance. It's currently at version 0.93.