Informal online seminars or reading groups via videoconferencing? Does the following exist, and if not, does anyone besides me wish it did? A web site where a mathematician (say) could find other mathematicians who want to study the same book or paper, and arrange to meet via videoconference, and run their own informal seminar around that topic, and then disband when they're done.
The reason I ask is that I have plenty of material I'd like to learn this way. I have an interesting job in industry, with occasional mathematical challenges, but there are more topics I would like to learn outside of work, and that I didn't get a chance to cover in grad school. I live near two major universities, and they do offer some interesting courses I could audit, but I'd rather focus on the exact material I want to learn.
One reason such a site might be successful (or, more precisely, popular) is that I expect academic mathematicians and grad students would find great use for it as well, since it can be hard to branch out to new areas on your own, if your own department doesn't overlap enough. If you have a deep professional network in the math community, you are probably better off, but not everyone is in that position for one reason or another.
I expect such a site, if it doesn't exist today, wouldn't be hard to create. If the videoconferencing portion was done outside the site using Skype or another service, then the site would just have to match up the groups, and maybe offer other features to support a running seminar. It would basically be a specialized social networking site, or maybe even an application within an existing site like Facebook.
UPDATE 1: The more I think about it, the more I think meetup.com is a good fit too, since it already has the calendaring and rsvp-ing features. The downside is that each seminar would probably have to be a separate meetup, and meetup.com is sort of tied to geography.
UPDATE 2: I have spent a lot of time looking for good, free, non-sleazy video streaming services, and I ended up at livestream.com. They have a couple key features. They offer a free plan with a reasonable business model behind it (the "freemium" model with a paid tier and a free tier that shows ads to viewers, though I haven't seen what these ads look like yet). They have Mac and Windows client software that lets you use your webcam OR stream your desktop, the latter of which I think is very important as a replacement for a physical blackboard or whiteboard. The other piece I have been researching is drawing on the screen with a pen. The best solution I have found is slightly sub-optimal: this pen or this one. They are similar in that they are standard ink pens with a wireless transmitter, and you clip a receiver to the paper and then write normally. The notes are either saved as images (the primary use case), or if you have a recent version of Windows, it can activate the pen features of the OS and you can draw directly into OneNote and other similar programs. This latter is what I want -- the seminar speaker can write on a paid of paper, and the remote attendees will see the writing rendered in real time on their screen. The problems are several: the pens cost at least $50-$60, I haven't tested them, reviews are scarce and Mac reviews are even scarcer. Most digital pen enthusiasm is focused on the Livescribe pen, which is strictly for saving notes and audio for later, and does not transmit in real time. Lastly, I also found this chat software that supports audio and a whiteboard, and which is free. By the way, these pens all look really great for students, especially the Livescribe, because it syncs the audio with the notes, so you can play it back and know what was being said when something was written. And you can convert a session to Flash and upload it as a "pencast". I think pencasts could really be wonderful for mathematics presentations that are recorded offline and then uploaded for an audience to learn from asynchronously.
 A: The idea is very natural. 
We are regularly using webex (webex.com) for seminars and interactive lectures. It works very well but it is commercial. Webinar system for math should have 1. good scalable write board 2. option to share .pdf and to write on it 3. good recording for the meeting.  In low price or free systems  something is lacking.      
A: Last (academic) year I organised a video seminar. We didn't use Skype; we used some gadgetry called an "Access Grid room". This is some kit which is installed in a lot of universities in the UK and, I believe, across the world. Ultimately at least 20 universities in the UK were capable of connecting to one another and listening to the speaker.
http://www2.imperial.ac.uk/~buzzard/maths/videoseminar.html is a link to the seminar.
Here is a summary of how it worked: the speaker had to be in an Access Grid room. If the audience wanted to be seen and heard, as well as seeing and hearing, they also had to be in an Access Grid room, although this Access Grid room did not of course have to be the same one as the speaker. Access Grid rooms with people in were videoed and the streaming video from each room, plus sound, was broadcast in each of the other rooms (typically on the back wall, so the speaker could see his audience, and then, behind them, 10 more audiences). However, if you just wanted to lurk, you could do so from the comfort of your own home, as long as you had a broadband connection. This is perhaps not quite what the questioner had in mind---for example with skype you'd presumably be able to contribute from home---but it's just a data point to suggest that stuff the questioner is suggesting is already happening. 
A: look @ this website http://www.twiddla.com/ . you can use it with your tablet and also has latex capability as well
A: I do not see much of a point in seeing people's faces and with full video either resolution is low, or jittering or the badnwidth is huge. So the solution is to have a simultanous voice and shared white board, which should be controlled by the individual elctronic tablet devices (mouse is not good for drawing). The electronic pen on the tablet is giving a one-dimensional (possibly with described path width, color etc.) trace, so if one just transmits the changes with the same resolution it amounts to many orders of magnitude lower data bandwidth. 
It would be nice to have (and it is easy for professionals to create such a thing) a software for recording the lectures in a format which would include just the voice and the electronic pen on tablet path with the timing information on the movement of the pen path synchronous with the voice. I do not know which voice and whiteboard formats would be based on such synchonous timing information interspersed with audio and new path information. Of course one should be able to erase, and do the recording in chunks. Application which would be a viewer for such recording would be like a video but having a far higher resolution with smaller memory used as only 1-d information is added and most of the rest of the picture is static; and the voice is not that big of a deal. This kind of setup is a must for mathematical community. Many mathematical centers do big efforts to make videos where the focusing, jittering, shade and other problems as well as the mere size of video files make it impossible to be in good resolution and not hugely expensive or of big memory size. The sound + timed chalk/pen-path solution is much cheaper and scalable to hi resolution at present level of resources and technology. 
A: Consider a version of your idea that does not use the word video.
Back in 1997, I contemplated holding a general algebra seminar using IRC (Internet Relay Chat).  While I thought that the ASCII-only medium would be limiting, at that time I saw that it could be combined with .jpg sharing and using a common web page to communicate ideas that weren't nicely formed from an ASCII character set.  I decided that with sufficient preparation and willingness to work with the technology, I could pull off some decent presentations, since much of what I wanted to say was with words, supplemented with a few diagrams.
In this millenium, the software gives some more power and flexibility of expression, but unless you want to talk primarily in pictures, consider how your seminar might go if your bandwidth was limited to, say, sending 8 pictures.  This can help one focus on the goal of communication, rather than seeing what degree of flash and coolness can be accomplished.
The other point of the consideration comes from the fact that chat communities formed quickly and segregated quickly, so I was able to go to #math (an IRC channel) and look for a target audience more easily.  I think the issues are : 1) Finding an online community, and 2) getting enough of that community to agree on common goals for such a project.  What technology you use and what shortcomings it may have will be minor obstacles to a group that dedicates itself to sharing and creating knowledge.
Regarding issue 1), there are enough communities now and means of asking for participation that finding such a group should not take long.  USENET news, IRC, social networking sites, mailing lists, blogs and forums are prevalent; spending a couple hours with these should get you underway towards building the specific community you want.  Issue 2) can be resolved by the degree of creativity, flexibility, and fun you bring to the process.
Here's a thought:  Can Math Overflow have or support such a category like Announcements?  It could be a button next to the Questions button, so that people could post brief announcements and reading group requests?  Off to meta.mathoverflow.
Gerhard "Ask Me About System Design" Paseman, 2010.03.24
A: it seems that google wave will be capable of something like this in the future (hopefully not too distant), maybe even already... My thoughts are that you could have one person giving a lecture live via video and have discussion underneath, but maybe this is just ustream... so you should prolly check that out...
A: The software Elluminate Live is quite impressive, permitting simultaneous text-chatting,
video, audio, and a nice whiteboard that onto which all participants can write and deposit images.
All aspects are optional, i.e., 
some can participate only via text, some also with video and/or audio, etc.
It is—alas—a commercial product.  No doubt analogous software either is already or soon will be
available free.
