Github is also quite good (
http://developer.github.com). They are straight forward docs for developer, and the prospect that the complete site is on github itself is a +1. I had some troubles with Cors support recently, and found it part of the spec within two clicks from their main portal. Try to
a) Keep headings readable (nothing fancy) and un-ambiguous. Don't make me wonder whether I have to go to Authorization or Authentication for instance.
b) Document everything. Even small nuances that you feel are implied. The API specs should change before your API changes.
c) Give me an archived version somewhere. This is easier if you just release your docs as a repo on github. This really helps people out. I ended up converting the parse.com documentation to markdown just to get an offline, readable version for myself.