VLC media player - http://www.videolan.org/vlc/ - general media player that plays just about anything.
Perian codecs for Quicktime - http://www.perian.org - plugins to add functionality to Quicktime for playing additional audio and video formats.
Quicktime audio problems, Perian, VLC - http://discussions.apple.com/message.jspa?messageID=9013669 - read note about uninstalling prior DIVX support before installing Perian.
I have installed Perian, and I get a message box saying:
The document "file.flv" could not
be opened. The movie's file format isn't
Quick Time Player 10.1 (501.22)
Also, I have read the Perian note about uninstalling prior DIVX support, but the note does not explain HOW to uninstall the listed items.
Take a look at the “range” CGI parameter in the HTTP request for the flv file. If it doesn't start with 0, it won't include the necessary header info that players can recognize to demux the stream. It seems that YouTube has recently started segmenting the flvs, and concatenating them in the Flash player.