XRDS-Simple is an important piece of the DiSo project. From the XRDS-Simple spec::

XRDS-Simple provides a format and workflow for the discovery of resources metadata, and other linked resources. As web services continue to grow, applications utilize a wider range of web services and resources across multiple providers. XRDS-Simple allows providers to document their resources in a machine-readable way, which can be automatically discovered by consumer applications.

So, check it out: http://drupal.org/project/xrds_simple .