CORBA has IDL; SOAP has WSDL. Developers often ask for similar capabilites for HTTP-based, "RESTful" services. While WSDL does claim support for HTTP, isn't well-positioned to take advantage of HTTP's features, nor to encourage good practice.
There are a lot of proposals in this space, but WADL is the most mature and capable. More thoughts here.
wadl_documentation.xsl is an XSL stylesheet that transforms WADL into human-readable documentation, one of many uses of a Web description format.
See an example of its output at <http://mnot.github.com/wadl_stylesheets/>.
Note that EXSLT node-set support is required, so this stylesheet will not work natively in some browsers. Try Saxon or xsltproc.
uri.xsd is a library of XML type restrictions that correspond to different components of a URI, so they can be accurately described (e.g., in a WADL param element).
Feedback is preferred on the W3C http description list.