Online webservice API/reference guide - What software application do you use to document your webservice/API technical specification/reference guide?
A nice solution is to build the documentation yourself using xml documentation comments and then creating web pages from the resulting xml.
MSDN article on using xml comments.
There are a few libraries out there that make it simple for you to create documentation for your api.
Atomineer is very good and can create documentation in many different formats to meet your needs.
With Web API documentation is rolled into your project with a nice UI to expose it. This also uses xml documentation and you can add your own details to that which comes out of the box.
[Broken link, am trying to find it again.] Heres a good blog post on this method api documentaion