Zendesk - Zendesk is a beautiful, lightweight help-desk solution.
Issuetrak - Issue tracking software for help desk and customer service
Freshdesk - Freshdesk is a cloud-based customer support software that lets you support customers through traditional channels like phone and email, social channels like Facebook and Twitter, and your own branded community
Deskun - Deskun is a Help Desk system inside Gmail. It enables you to support customers directly from your mailbox, manage projects, and use free features for Gmail.
HelpScout - Help Scout is a simple, straightforward way to provide excellent support
Kapdesk - Kapdesk offers support teams with a helpdesk software to streamline support tasks.