If you're looking for a tool that will allow you to produce professional looking video, then you should take a look at Cinelerra. The interface looks fairly untidy, but you soon get used to it and the results that are possible more than make up for this fact. There is also a new version of Cinelerra which is being developed by the Cinelerra community, which is called Lumiere, although there are no releases yet.
Mostly, it seems that people use a combination of Cinelerra, The GIMP and FFMPEG to create video under Linux, along with Kino / dvgrab to take video from a digital camcorder.
A great place to get music from that you can use in your videos is Magnatune, where you can buy music from relatively unknown artists for a fair price. The artists also get a fairer deal than they would do under a mainstream label. Due to the way the music is licensed, you can use the music in your own personal projects (under the Creative Commons License).
For Debian users, the easiest way to get started with Cinelerra is to use the Debian Multimedia repository. You may also want to compile Cinelerra yourself but this requires more technical know-how.
Recordmydesktop is a good tool to use for creating screencasts. Apparently Cinelerra can also record the desktop but I haven't tried this as yet. If you're recording from an application that uses OpenGL, then glc is worth a look, although you'll need to run it from the terminal and spend a little time reading how it works.
Update! Please see screencasting guide for glc here