Let’s start with Xtranormal, a site that begins to bring simple animation projects into the realm of the hobbyist:Vodpod videos no longer available.
Fun to play with but it would be awesome if you could sync in your own voices instead of sticking with the limited CG voices.
Also worth a look is ZunaVision out of Stanford University. This is another way to place ads or other graphics onto existing video in a less obstusive manner:
Ah, the internet. Cool stuff.