Berbee has some example XML services that are free to use at http://www.berbee.com/public/berbeesoftware/XMLFree.aspx
The problem is that one cannot simply add a service to the url services: a new service always replaces the current service. That why you have to create a custom menu.xml file on your webserver if you still want to use VoiceView Express. (The instructions on Berbee's site are for full blown Call Manager.)

Here's an example menu that I use in demo's for the UC520

Has anyone successfully setup a Berbee free XML with Call Manager Express?

http://www.berbee.com/public/berbeesoftware/XMLFree.aspx

I am interested in developing applications that communicate and integrate with the UC 500 system for SMBs. Can anyone tell me what is required to do this or is it possible?

XML Services

Free XML Services for Cisco IP Phones
Berbee, a Cisco Partner, has several free apps such as stock tickers and Weather updates.