SOAP::Lite, Perl module, kicks serious arse. Client and server implemented for SOAP and xmlrpc, with a really good interface -- by which I mean: You can do things as difficult and high-end as you need, but if you're doing simple stuff the interface scales back down with you, getting easier and easier. Impressed. (We've just demoed a three part system communicating with xmlrpc and SOAP, with php and AppleScript frontends. Sweet.)