Mapzen is also excellent (although definitely less mature than Mapbox), and a huge chunk of their stack is open source.
- web and mobile SDKs: http://github.com/mapzen/
- map rendering library: http://github.com/tangrams
- map tiles: https://github.com/tilezen/
- routing/navigation: https://github.com/valhalla/
- public transit: https://github.com/transitland/
- search/geocoding: http://github.com/pelias
- gazetteer: https://github.com/whosonfirst
FWIW, we do have a few private repos, to protect some ops internals as well as upcoming blog posts.
(I work at Mapzen -- and am also a happy user of Mapbox's open-source libraries.)
Everything I've seen so far has been clean and pretty easy to follow. Really appreciate the work you all do.
I'm a very happy Mapzen customer.