Zendesk - Zendesk is a beautiful, lightweight help-desk solution.
osTicket - osTicket is a widely-used open source support ticket system.
Raiseaticket - Raiseaticket is a 100% free cloud based helpdesk for customer service and support. The saas provides portal, workflows, insights to manage communications for the support and service process.
Zendo - Zendo allows you to sell productized, subscription and custom services. Collaborate and collect payments. Create quotes and invoices. Eliminate multiple tools and enjoy the simplicity.
Best Support System - Best Support System is self-hosted support ticket application.It handle support ticket, manage email ticket, collect any payment by paypal.
Usedesk - Usedesk is a helpdesk platform that helps automate your customer service, keep your business in sync, and your clients happy - all in one single window.