I’ve been playing with the Aviary Online Garage Band style web application called Audio Editor and have been cranking out some neat quick atmospheric items. While it takes a bit of fiddling to get good results, if you’re looking to craft a fifteen second introduction theme, like the one on Howard Rheingold’s videos, then this is a free way to do it. If you spend $25 to $50 on a sample kit you could put together a pretty decent intros and outros for videos or interludes.
It’s fairly intuitive, drag a track to the timeline then add another couple. Add effects, twiddle virtual knobs, and away you go.
Another similar project, although definitely slanted towards electronic music, is hobnox. In some ways hobnox seems more organic, plug the tone bank or 808 clone into a few pedals and dump it into the mixer, then the amp.
So you can add a little pizazz to your videos, which if the content is good, you’ll be able to make them closer to a professional production.