Physical attendence at a lecture has its advantages. The quality of the experience is infinitely higher, there is the option of interactivity, realtime outcomes have the spice of unpredictability, and often there are free snacks.

However stored lectures are certainly convenient, plus you can surf -- watch a few minutes of lecture A, a few of B, etc., etc., - - before committing to any one in particular. To the degree that a scorching first five minutes is diagnostic of the whole this approach pretty much guarantees a quality experience. You do need to have a pretty deep pool of choices, but there is good news out there on that.

Perhaps the global leader in this field is Cambridge University's magnificent Videolectures.net.  MIT has by far the richest archives locally, supporting three big stored video libraries: MIT Tech TV,  MITWorld,  and OpenCourseWare.   (Selections from these archives also appear on iTunes U and YouYube.)

MIT Tech TV is MIT's YouTube. Incredibly varied.  Here is one example, not that one example could possibly be typical.   There is often something interesting here, which links to a collection of the videos MIT has used to illustrate its press releases..

MITWorld carries several hundred videos of events at MIT, including hundreds of lectures on science and technology. Lectures tend to be addressed to a general but sophisticated public, are usually about important subjects, and are often delivered by the key investigators of the field. MITWorld is also noteworthy in that someone has bothered to write comprehensive abstracts for most of the entries, a feature that almost all other archives lack. I might recommend Grant Wilson's talk on Nanofabrication and Stephen Chu's overview of thermodynamics.

Finally there is the famous OpenCourseWare, which contains actual lectures from real classes. Perhaps the most famous of these are the physics lectures given by Walter Lewin.  Dozens of colleges around the world have followed MIT's example (including Tufts and UMass), although not many offer a video lecture archive like MIT's.

The Media Lab has a small archive of its own events, as does The Computation for Design and Optimization Program.

MIT is conducting a couple of interesting experiments with the medium. Note especially the Lecture Browser -- a project of the Spoken Language Systems Group. A subset of lectures were copied from the above libraries, run through a speech recognizer, and the resulting text indexed. Anyone wishing to locate a specific term or usage in a lecture needs only to type that term into the search field. The engine will pull up all the lectures in which that term appears, presenting you with an audio copy and the transcribed text, starting both versions at the point where the term of interest appears.

Other academic institutions in the area have much smaller archives, at least on the topics of interest here. As of January 08 there were about twenty videos tagged as "science" (most recent - April 4, 2007) and eight tagged as "technology" (April 13, 2007) in harvard@home, Harvard's primary video depository. Other associated archives are scattered among the Radcliffe Institute, the Evolution and Theology of Cooperation Project, the Mind/Brain/Behavior Intiative, the (highly recommended) Observatory Night Lectures at the Harvard-Smithsonian Center for Astrophysics, and the Longwood Seminars and Science in the News Initiative, both at the Harvard Medical School. Like they say at Harvard, "Every tub on its own bottom". The Harvard School of Public Health has a few items.

Outside Harvard and MIT pickings are slim. BUniverse, BU's archive, is just getting off the ground. The topic category called "Science, Technology, and Math" contained ten lectures on March 29, 2008). (That said, when BU does record a talk it is often an exceptional event by someone really interesting, like Murray Gell- Mann or Freeman Dyson.) You can find a complete list of all videos by clicking on the 'search' link without entering a keyword in that field.   Recommended: Artificial Intelligence in Video Games with Ian Davis.   The Museum of Science has a small archive. Tufts' Wright Center for Science Education has some lectures on "cosmic evolution" here.

One reason why local institutions do not have their own libraries might be the presence of the WGBH Forum Network, a local consortium that records and serves events of interest sponsored by its members, most of whom are local. As of January 08, the WFN reported serving 2,000 lectures which in aggregate had been streamed about a million times. About 250 lectures are tagged with either "science" or "technology". There are good abstracts.

Dartmouth's archives are pretty thin, despite the campus' rep as a high-tech player. Recommended: a short video about the creation of the world's smallest steerable, untethered, mobile, robot.