osTicket

osTicket is a widely-used open source support ticket system. It seamlessly integrates inquiries created via email, phone and web-based forms into a simple easy-to-use multi-user web interface. Manage, organize and archive all your support requests and responses in one place while providing your customers with accountability and responsiveness they deserve.
osTicket is a help desk solution designed to help businesses of all sizes manage incoming support requests to streamline customer service operations. Key features of the platform include custom ticket queues, ticket filters, configurable help topics, ticket locking mechanism, ticket transfer, thread action, service level agreement plans, customer portal, search functionality, task management, and more among others. It also lets administrators send personalized automated responses to clients.