A while ago I had to work on a PHP support ticket system. This system was not WordPress related and very quickly, I got annoyed, since a lot of basic features I am used to where missing. For example, although this system has a templating mechanism, the menu is more or less hardcoded. To rename the “Home”-link you had to do edit the core.
Well, anyway. After I finished the job, I started to work on my own support ticket plugin for WordPress. I tried to design it the way, I would have loved it as a developer. The result is this plugin, which made it today into the WordPress repository.
It still misses some features, I want to add, but it already has a lot of nice features and is a fully operating support ticket system. Have a look, I would be more than happy, if you would use it to communicate with your clients.