WordPress supports Pingback and Trackback out of the box. Not familiar with that? Then check the explanation on the WordPress site or very easy explanation of both in German. The both things of course have some disadvantages. They can lead to some spam trackbacks. There is some solution, in a form of Plugin that validates whether pingback is valid, but the validation occurs only at posting time:(. However this is an nice start.
The technique besides it seems to be based on XML-RPC. Interesting is also how the RPC-Services are discovered. The is one RSD document for this purpose. So RSD seems to have similar goals with WSIL, but RSD is lightweight. Maybe that is the explanation of the fact, that RSD is going to be popular. I’m sure just because it very simple. While WSIL/WSDL describes the endpoint of the protocol as well as a description of messages that finely go across the wire, RSD just notes the endpoint and expects from the invoker a prior knowledge of how to implement the protocol.
Here is WordPress default RSD on my blog: http://alexander.holbreich.org/xmlrpc.php?rsd