DimDim is a very powerful web-meeting application, works great on all platforms we’ve tried. The latest Open Source release (3.5-beta) is very stable and was easy to install. The end-user experience when joining a meeting is much nicer than what WebEx offers.
Only limitation is that desktop sharing on Linux is not supported yet. It’s on their task list, we’re hoping it’s functional by end-of-year. Until then a combo of OpenVPN and x11vnc does the trick.