Server that layers some type of info/interaction on top of a 3rd party's "base".
Wiki Proxy, Wiki Mail, Annotation Systems
http://en.wikipedia.org/wiki/Proxy_server
LesOrchard:PersonalWebProxy
The Universal Inbox could be thought of this way as well.
Ian Bicking identifies some issues that come up in a WSGI context.
Py Thon-based ProxyServer-s http://xhaus.com/alan/python/proxies.html