Open Opus is a free, open database for classical music metadata. It was created as a backend for the players Concertmaster and Concertino. But it can be used in any classical music application. The data is completely open for anyone not only to use but to edit as well - it's a wiki-style free-to-edit database. Open Opus' software is also open sourced and everybody is invited to contribute with code or fork it.
Concertmaster and Concertino are open source classical music front-ends for Spotify and Apple Music, respectively. They consume Open Opus data and combine it with automated analysis of the Spotify/Apple Music own metadata in order to create an adequate user experience for classical music. It relies on crowdsourced curation as well for additional verification and refinement.