Free Help Desk System: HESK

HESK is a free help desk (also known as support ticket) system that is built with PHP/MySQL.

Using the application, anyone can submit tickets from a web-based interface & follow their status easily. Also, the admins will be able to sort any requests by priority & answer them without leaving any non-replied requests.

The admin-side has support for canned responses for faster replies, tickets can be auto-closed after x days & more.

There is also a full-featured knowledgebase, that supports unlimited categories, where users can find answers to their questions themselves.

P.S. HESK has a free & paid version where the only limitiation of the free version is a backlink.