XML-RPC (Score 4)

I'd definatly have a look at XML-RPC (http://www.xml-rpc.com/)

While implementations are not available in every language (of note, Java, Perl, and Python have implementations), it's simple enough to write your own easily.

I've writtern a few programs using it with Python and Delphi, with great results.

In essence, it's doing a procedure call, with the parameters and return values in an XML format, over HTTP. If you're famliar with both, it's dead simple to do, if not, it's a great excuse to learn :)