[vetinari]

Flash uses Sorenson Video 3, which is derivative of h263. The irony is, that Apple used to exclusively license the SV3 codec for Quicktime (they used to deliver trailers in mov + sv3 + qdm2) and Adobe later licensed the same codec for Flash (later also On2 VP6 and H.264).

The point is, that Quicktime has supported the video (except VP6) and audio codecs used by Flash for ages, it just does not support the flv container. If you remultiplex any youtube video into mov container, any stock Quicktime installation will be able to play it.