The ASP.NET Web API Help Page automatically generates help page content for the web APIs on your site.
"The ASP.NET Web API Help Page automatically generates help page content for the web APIs on your site… Everything generated by the help page is fully customizable using ASP.NET MVC and Razor. ASP.NET Web API Help Page is a great addition to any ASP.NET Web API project."
For more information on how to use the ASP.NET Web API Help Page, you can start with this tutorial from the Microsoft Web API site; more advanced usage scenarios are outlined at Yao Huang's blog on MSDN.