I started BowlSK when Google first released App Engine. I signed up right away, then had to figure out what to build. I had been looking at other apps/sites to track bowling information and none got as granular as I wanted. So I learned enough Python to get started and began hacking away. 7 (wow!) years later and it's a bit neglected on the coding end, but I and many others still use it for league every week.
eXist-db is an open source XML database. My contributions included rewriting the internal handling of URIs to use a dedicated URI class instead of strings, to enforce consistency regarding collection name encoding. Memory footprint and query speed were very important, necessitating multiple URI implementations - one for absolute paths, and a lighter weight one for relative paths.
The Rocksmith Custom Song Toolkit allows people to create custom songs that are playable in Rocksmith. Contributions included reverse engineering work on the binary song format, as well as a bunch of work on the client application to create custom song packages, and a converter from another popular music file format.