We use DimDim frequently, as the free version allows up to 25 users in a meeting. The DimDim Pro adds the ability to invite up to 50 users and record and playback the meeting, but that wasn't always a high priority for us.
Since DimDim has not yet released the code of their fork of the open source project, you can download the "pure" open source meeting tool upon which it was based, the BigBlueButton project. BigBlueButton is even available to download as a pre-configured Virtual Appliance that can be run in VMWare Player or VMWare Workstation (as well as VMWare ESX).
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