Hesk is free and rich with features, including unlimited categories, staff with customizable privileges, customizable fields, canned responses, fully-featured knowledgebase (FAQ), e-mail notification and much more.
This script can be easily translated to any language, has customizable header, footer and style files. Since it comes with open source code at no charge you can basically customize it to your specific needs as long as you can code in PHP (or hire someone to do it).

Listed in Customer Support php scripts category