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.
The Open Source edition of Dimdim is developed using open source components like the Red5 streaming media server, lighttpd Web server, CherryPy Web application framework, and others. The latest version of this edition is available as a platform-agnostic VMware virtual appliance or with a native Linux installer that's configured to run only on CentOS 4.5 (or Windows). CentOS is a fine choice for a server distro, but having it as a required dependency excludes a fair number of potential users. On the other hand, the virtual appliance, which is easy to deploy, tips the scales as a 1.3GB+ compressed download.