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Dimdim, the free, open-source web meeting company, has raised $6 million in a second round of funding.

The concept is pretty simple — like WebEx, but free. Dimdim offers all the basic tools that you’d expect from a web meeting service, including a collaborative whiteboard, desktop sharing, audio and video. It doesn’t have too many bells and whistles, and some of the technical kinks still need to be worked out; I’ve run into a few display and usability glitches myself. But even if Dimdim isn’t perfect, it works. And since you don’t have to pay to use it, that counts for a lot.

Chief executive DD Ganguly says Dimdim has been used by more than 500,000 people in more than 180 companies since it launched in private testing mode in 2007. What’s particularly exciting is how, as Ganguly says, Dimdim “democratizes” web meetings. Sure, the phrase reeks of hype, but it’s also accurate — without WebEx’s cost, Dimdim can take web meetings out of just the corporate world and make them a broader tool for communication. (My favorite example is still the person in Florida using Dimdim to teach English to Mexican immigrants before they come to the United States.)

I’ve been doing my own small part to promote the company, too. Whenever a PR person or CEO suggests using WebEx, I point out WebEx’s lack of Mac compatibility (what’s up with that, anyway?), and immediately suggest Dimdim as a free alternative. People are even starting to take me up on it.

The funding comes from existing investors Index Ventures, Nexus India Capital and Draper Richards, and follows Dimdim’s $2.4 million first round last year.

Ganguly says most of the new funding will go toward marketing. The Boston start-up’s next moves will also involve taking its technology into different markets, starting with video later this year. That way, people can take advantage of Dimdim’s video or chat services on different Web sites, for example, without having to create a full web meeting.

Dimdim plans to make money by offering a premium service. Advertising in the free version is also a possibility.

Update: As a WebEx spokesperson notes in the comments, I let my whining get a little over-the-top. According to the WebEx system requirements, the service does support Macs, but without remote access. I’m not sure if it’s remote access or some other issue in my case, but I’ve never been able to get WebEx meetings to work on my Macbook, and other Mac-owning VentureBeat writers have reported similar problems. In my limited experience WebEx has not been Mac-friendly, but I definitely overstated the case.