I don't know when SugarCRM, the open source CRM tool, first acquired a SOAP API but I missed it completely. I've used SugarCRM a few times, on quite a casual level, and only fell across the SOAP API because I needed it.
The first thing to note is that the SOAP API for SugarCRM is advertised as being incompatible with the PHP 5 SOAP client implementation, and they recommend you use nusoap. However this isn't true and I found it worked fine with PHP 5 in non-WSDL mode.
The second thing to note is that the documentation is rubbish. The best I could find is on their developer wiki and it isn't enough to write an application with. I had to do a fair amount of fiddling around on the server side to understand when I was making mistakes which isn't ideal for remote system calls! They do, however, have very good inline documentation so I ran PHPDocumentor over the file which had the stuff I was dealing with in it. I can't find an equivalent online so you can see my copy over here
Also if you go to your own sugar installation and append /soap.php to the URL, you will see some information there about formats plus a link to the WSDL. The WSDL isn't very human-readable but the top bit of it defines the custom data formats like "name_value_list" and shows you how to assemble them to submit to SOAP. If I can work out a good way of presenting that information I'll add it somewhere and link to it.
There will also be a follow-up with the script that worked for me. If this helps, or you have any questions, then add a comment please :)