I’m sure this is one of those tools which almost everyone knows about, but it’s useful enough that I’d post it if only a single person benefited: Perian is a component for which adds native Quicktime support for, notably, FLV (and a bunch of other formats nobody really cares about).
This means you can play those downloaded FLV files in VDMX, 3L… any software which uses the Quicktime framework for playing video (feel free to chime in with the ones I’ve missed). So now you can youtube VJ in your favorite software.