This has been a popularly viewed post but sadly little input! I did some research and found a few that seem to be the 'standard' best.
Be sure to check out the top 100
projects on sourceforge!
looks like the most developed conferencing interface. A little tricky to setup but of a very high standard, check it out.
is specifically for those who live in the Yahoo messaging world. Fully featured.
is very popular but the stable versions do not offer video support. You can try and build it yourself, or run a win version in wine. You may have many problems here!
is the new name for Gnome-meeting and is default with many distros.
I haven't found anything for google/gmail or MSN networks.