Software

Access Grid
ConferenceXP How to install and configure ConferenceXP Client
EVO -Enabling Virtual Organisations - Video Collaboration Tool
Pidgin - Instant Messaging Client (it supports most IM services such as GTalk, MSN, Yahoo, AOL, Jabber, ICQ, etc.)
Skype - very popular VoIP/Videoconferencing system (some concerns in its quality and security)
VSee - small scale (3 - 6 sites) videoconferencing system (very good application sharing function)