Apiary is a web based repository for API documentation that also includes server mocking, code samples, automated testing, and GitHub sync. It's free for most needs including both public and private API documentation.
Both 3scale and Apiary are great tools for managing APIs. 3scale has some great features to offer such as log tracking, single sign-on, custom branding, analytics etc. 3scale has two plans - a Pro plan that is priced at $750 perAPI Management Services