Dimdim, the (mostly) free, open source competitor to online meeting companies WebEx and GoToMeeting, is launching its service in public testing mode today.

Chief Marketing Officer Steve Chazin gave me a demonstration of Dimdim’s free service, and it has the basic functions that you’d want for an online meeting: a collaborative whiteboard, desktop sharing, slideshows and audio and video. The video and audio quality aren’t top-notch, but they work. The selling point, of course, isn’t the quality or innovation of the service — as long as Dimdim is functional, the fact that it’s free makes the service pretty attractive. Also, unlike other meeting services, you don’t have to install anything to join a meeting, not even a browser plug-in.

Dimdim started an invite-only test last fall. Since then, the company says more than 375,000 people in 165 companies have participated in Dimdim meetings. What’s really exciting about the Burlington, Mass. company is that it helps widen the market for online meetings — not just to smaller companies that don’t want to pay for WebEx, but also to groups outside of the corporate world. For example, someone in Florida is using Dimdim to teach English to Mexican immigrants before they arrive in the United States.

Back when we first wrote about Dimdim a year ago, we had one big question: How is the company going to make money? The answer is a combination of advertising on the free service, as well as charging for premium and enterprise-level products. The free version is already pretty good; you can host up to 20 people in a meeting. The fees for Dimdim Premium start at $99 per year and cover custom branding and meetings with up to 100 people.

Update: In the comments, Chazin says that advertising on the free service is, for now, just a possibility.

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