DimDim is a web conferencing product popular for education. This is because it offers a free service for conferences with up to 20 participants. It also claims to require no software download and be open source. However, it uses Adobe Flash for the audio and video, so that needs to already be installed on the computer used. I found DimDim very easy to sign up for and start a conference with. It has the usual whiteboard, desktop and presentation sharing features of such products. One interesting feature is web co-browsing, where the presenter selects a web page and it is displayed on the participants screens, scrolling synchronised with the presenter's screen.
Co-browsing works very well with HTML Slidy presentations, such as my "Learning to lower costs and carbon emissions with ICT" (slides). This could be very useful for bandwidth efficient presentations, as the Slidy slides use a lot less storage than the typical Powerpoint or OpenOffice.Org presentation. With the video frame rate turned down (or off) and the audio reduced to telephone quality, the web conference would use very little bandwidth. USQ's ICE open source e-learning content creation system produces Slidy as part of a course package, which would then work well with DimDim.
One catch is that, for security reasons, DimDim does not work with web sites which require you to enter a user-id and password. I found that this stopped access to Moodle courses, even those which allow access without a user id (I will ask DimDim to fix this). Another limitation is that the web browsing is not recorded along with the audio, video and other content.