At Mapzen, all our code is open source, from front-end UIs to back-end "glue." Have a look at:

- 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.)