What's the awesomest (online) product documentation you've ever used?

gistfile1.md

https://www.djangoproject.com/

http://www.springsource.org/documentation

http://static.springsource.org/spring/docs/3.1.x/spring-framework-reference/html/

http://www.djangobook.com/en/2.0/index.html

http://redis.io/documentation

http://www.djangobook.com/en/2.0/index.html

http://www.squid-cache.org/Doc/config/ # look at that versioning info though somewhat hard to navigate and find what you need
http://haproxy.1wt.eu/download/1.5/doc/configuration.txt # simple, very well-written, easily searchable (with find on page)
https://github.com/kr/beanstalkd/blob/master/doc/protocol.txt # the core of how this (simple) thing works but no-nonsense
http://quartz-scheduler.org/documentation
http://www.postgresql.org/docs/9.2/interactive/index.html # clear, well-organised, comprehensive
http://search.cpan.org/dist/Catalyst-Manual/ # Catalyst, CPAN generally