very interesting let me add somme that i like:

first Marratech: very high-end systeme based on java , completely cross platform , but you got to have access to a server or at least a machine that can act like one. the free version is limited to 5 people but works great http://www.marratech.com/free/
(don't be distracted by the insanely bad web site, it's a good software)

Next the newest kid on the block, Festoon, they use a mix of the jabber protocol http://www.xmpp.org/ and skype integration to create a neat webconferencing solution for free.

BTW: xmpp IS the big news of this year pushed by google and other big names in open standards world, it's going to be invading the workplace this year. Fantastic project combining it with VoIP will create a new layer for internet applications. I recommend this short introduction by B. Mann
http://www.bmannconsulting.com/blog/bmann/2006-will-be-the-year-of-xmpp

Some really interesting time ahead when we put this in the perspective that Microsoft will have a built-in web conferencing solution in its upcoming Office 2007