Dimdim combines whiteboarding, screen sharing, and text/video chat all under one roof.
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A finished canvas can be downloaded or saved as a .PNG file. Basic services are free, but professional-level features (SSL access, permissions, support calls, saving as more sophisticated document formats) are available for sliding fees. Voice and video chat are also available through the tokbox.com affiliate service.
Despite the silly name, Dimdim corrals together an amazing number of meeting and collaboration tools in one place. It's primarily a Web meeting system rather than a collaboration system, but there are just enough collaborative elements (whiteboard, for instance) to make it worth mentioning here. The free version supports meetings with up to 20 people at once, and the for-pay editions of the service allow much bigger audiences.
Even an abbreviated list of all the services available through Dimdim is an eye-opener: audio/video sharing through Webcams; live screen sharing (this requires a plugin, though); synchronized Web browsing; whiteboarding; document sharing; the ability to record meetings; and even dial-in support for people who just want to listen and talk by phone.
Obviously all of this power comes with a few technical limitations. Flash is required and only Internet Explorer and Firefox are supported. Also, the only kinds of documents you can share are PDFs or PowerPoint documents; Word isn't directly supported. One possible workaround would be to upload a PDF-converted document or just point people towards a Web page with the text.
If you want to run your own Dimdim server, you can grab a copy of the source code and do so.
Another oddly named site with a great mix of features, Yugma centers around being able to collaborate from your own desktop in real time. In other words, you're not confined to uploading a document somewhere and sharing that: you can point to an application running on your own machine and share it. It runs on all three major platforms -- Windows, Mac, and Linux -- and the free version works with up to 20 people. Other editions go from $15 a month to $180 a month.