Dimdim Web conferencing software, which competes with services like WebEx and GoToMeeting, provides almost all the important features you need for conducting a conference over the Web. It's available in three flavors -- a feature-limited but usable Web-based free version, a no-holds-barred fee-based Enterprise version, and an almost Enterprise clone Open Source Community Edition that you can host in your network. I tested the Open Source edition, using it to host conferences on an intranet and over the Internet, and it works fairly well for a beta release.
Have you ever wanted to attend a webinar but quickly discovered that the web-conference service doesn't even support your operating system? This has happened to me several times and it irks me greatly! I always make sure to tell the company that they should switch their web-conferencing service to one that supports multiple platforms including Linux.