I've created a multiplatform desktop webservices client, (REST protocol)
Nowadays is quite limited because the webservices in Moodle 2.0 have some limitations
I.e, to update an user you must know his userid and there is no way to get a list of users (via webservices) with a username, idnumber, etc...
You can download the client here:http://sites.google.com/site/mooconsole/
The app is pluggable, you can create a XML file describing a webservice required data in order to add your custom webservices in Moodle and test it in this app.
I've created this app in my spare time so the code (python) doesn't look so good