Lychee
When I started photographing I wanted a good program to showcase my photos, which was when I found Lychee, a webapp written by Tobias Reich. The program worked really well, was easy to setup and faster than I could imagine.
But it lacked several features I really needed, like showing videos, having multiple users etc.
So when I found an incomplete video feature pull request I figured I could continue work on that and added the remaining features, like thumbnail creation, uploading from the browser etc.
I also found the default session handling a bit lacking, since it would log out after a short while, so I added a cookie and some more session handling which means that the session length is now configurable through the database.