Simple Object Access Protocol
See http://www.w3.org/TR/SOAP/ which covers the SOAP standard. A new version of SOAP, 1.2, was announced during 2003. The new version is quite a bit more complex, allowing various protocols, supporting attachments, email bindings, etc.
Related material is available at the Wiki pages for TclSOAP and WS, and also [L1 ].
XMLSpy [L2 ] "has a very cool function in it to test/debug SOAP", according to David Bigelow.
Additional Note - the Cool Function in XMLSpy is AWSOME - but not worth the US$1,000.00 price tag. Surely there is something that can be done to do the same thing in Native Tcl... - Dave...