The KDE Community
It provides client-side support for the XML-RPC protocol.
This library contains simple XML-RPC Client support. It is used mainly by the egroupware module of kdepim, but is a complete client and is quite easy to use. Only one interface is exposed to the world, kxmlrpcclient/client.h and of that interface, you only need to use 3 methods: setUrl, setUserAgent, and call.
A small note on authentication. If you will be accessing an XML-RPC server that uses HTTP-AUTH, simply set the user and pass in the KURL. Digest authentication is more complicated and requires the use of three utility methods: enableDigestAuth, disableDigestAuth, and digestAuth.
kxmlrpcclient development is discussed on the firstname.lastname@example.org mailing list.