Most of websites have a help desk, especially online stores, help desk helps you receive, track customer tickets and manage knowledgebase articles. It reduces a lot of labor workload and allows you to answer to clients' questions timely. Besides, a good help desk also increases your company's reputation and cultivate long-lasting relationships with your clients.
Yesterday, I came accross a free piece of hosted help desk help script, HESK. HESK is a fully-featured help desk, it helps you manage customer tickets and knowledgebase, and really easy to use. Besides, it is written in PHP/MySQL, you can install it on linux servers or windows servers. It's simple, but it is powerful, the key features of hesk including web-based interface, spam prevention, automated email notification, advanced search capabilities and multiple language support.