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.
Zoho Desk - Industry's first context-aware Helpdesk Software
Kapdesk - Kapdesk offers support teams with a helpdesk software to streamline support tasks.
Salesforce Service Cloud - Service Cloud integrates customer service with social networks and provides a self-service customer portal and knowledge base system.