Encourage your clients to read previous issues or submit their own tickets. For your clients: There is a side panel option, knowledgebase searches, the ability to upload files for attachment, sorting/filtering of organized issues, the ability to vote on issues or leave comments, and more.
For the customer service professional:
There's automatic HTML email notifications (to either the customer or the agent, a WYSIWYG text editor, custom fields, custom security levels, upload manager. There are many conveniences, quick icons and automated features (our customer support department uses our own help desk, so we want it to be very convenient). Still includes contact list, reports, inter-agent emailing, To-do list, and more tools. Free support, install, ASP source code & future version upgrade are included. (Visit for demo.)
V. 6.0 has multi-level categories (Main level, sub categories, and sub-sub categories.), unlimited number categories in each level.