osTicket - osTicket is a widely-used open source support ticket system.
TeamViewer - TeamViewer lets you establish a connection to any PC or server within just a few seconds.
Zammad - The Zammad - Help Desk and Ticket System - Community Page
mRemoteNG - mRemoteNG is a fork of mRemote, an open source, tabbed, multi-protocol, remote connections manager.
Zendesk - Zendesk is a beautiful, lightweight help-desk solution.
join.me - Instant screen sharing. Instant Aha!