SOAP

A more complex version of XmlRpc. (OK that's a bit too terse, it's a more generalized model, providing transport-protocol-independence, among other things...)

http://www.soapware.org/

I thought (2002-04-11-f) that Zope didn't have much SOAP support, but there are some articles talking about the ZSI.

A Sam Ruby Jul'02 piece on combining ReST and SOAP.

SOAP should allow transport over SMTP (ASynch Messaging), Jabber, etc. - but I believe you should not assume that a given toolkit will actually do so.

I'm not even sure how good the true cross-vendor InterOp is for SOAP.

A July'02 article compared the performance of XmlRpc, SOAP, and CORBA (using Python).

Mark Nottingham on SOAP as format instead of protocol.