Screencasts are like geek pr0n - and great advertising. If you haven't seen a screencast, picture watching someone who is really good at programming showing you a neat trick. They are incredibly good an pulling together a lot of ideas and seeing how something is actually done, rather than a list of features of what is can do.

This is true for open-source languages and applications that are trying to attract developers and users. Especially since documentations is notoriously difficult to write well, and therefore very scarce. Making a screencast of you using your tool/language/etc. is easy and fun (for developers).

It's also a great training tool and can provide very good tool for teaching users, where they can later review and get a better idea of how to use a feature than a description like "open the 'File' menu, select the 'Go to...' item... yadda yadda".

They're relatively easy to make. There are packages for Mac OS X such as Snapz Pro and for Windows options like Camtasia Studio or the free Windows media encoder.

Check out the Technorati listing of screencasts.

A couple stars:
The recently popular (and popularizing) Ruby on Rails screencasts, Mochikit (Mochikit Introduction Screencast), Django.