Whereby (formerly appear.in) - Collaborate from anywhere - video meetings with no downloads
Empathy - Apps/Empathy - GNOME Wiki!
Around.co - Around.co is an app that provides video calls to remote teams all around the world.
Pidgin - Pidgin is an easy to use and free chat client used by millions. Connect to AIM, MSN, Yahoo, and more chat networks all at once.
Remote Meeting - Remote Meeting with Video, Voice, Chat and Screen Sharing
Trillian - Trillian is a decentralized and federated instant messaging platform that lets your whole company send private and group messages, keep tabs on what co-workers are doing, share files, and much more.