http://datamade.us/blog/better-living-through-documentation/
A thorough list of recommendations on how to write developer-friendly documentation (and best practices). Contains many useful tips e.g. "don't put important content directly before or after a code snippet", "avoid the word 'just'" and "make sure code examples are okay to copy/paste".
http://openlayers.org/en/v3.8.2/examples/cluster.html
Executes the "easy copy/paste of code" mantra extremely well. Code is copied to clipboard or JSFiddle with a single click. The latter is extremely useful for doing quick, one-off experiments.
https://api.open.fec.gov/developers
Crème de la crème of API documentation. Contains executable code, response examples, foldable code blocks, concise and to the pont text, enthusiastic calls for participation, etc.
mapschool explains geo in an extremely basic, and therefore accessible, way. This is the level of complexity/simplicity that the Best Practice docs should strive for to achieve adoption by non-geo developers.
http://www.codeforamerica.org/ https://www.whitehouse.gov/digital/united-states-digital-service https://18f.gsa.gov/ https://gds.blog.gov.uk/
Are all great examples of large (non-commercial) organizations that engage with developers by building great tools and writing superb documentation, but also by opening their process to external contributions (see e.g. this and this) and being positively outspoken about it.
Noteworthy examples of documentation are U.S. Digital Service Playbook by USDS and U.S. Federal Web Design Standards by 18F. Both are open to contributions through GitHub.