Dimdim.com offers free web video conferences for up to 20 participants. The service is fairly simple to use, but highly customizable. Invitees need a browser with Flash support, but do not need a Dimdim account to participate.

In under ten minutes, the presenter can establish a meeting that includes screen/file sharing, a whiteboard, and group browsing, alongside public and private chat rooms. Scheduling and inviting people to the conference is done via a widget that emails details (which even includes a call-in phone bridge) to all participants. The conference can be recorded and subsequently embedded into any web page.

UPDATE: Dimdim.com was bought by salesforce.com in spring 2011. This service is not available after March 15, 2011.