As long as we're talking about OS 9, Dr. Dave alerted me to a couple of Japanese-authored apps for viewing Youtube and flv files in 9.2.2 (I didn't test these in earlier systems). The instructions and interface are written in Japanese, so it's a little confusing if you don't speak it, but I managed to more or less work it out.
(UPDATE: The YTBrowser9 link below is an outdated version. Try this new version here.)
They're called YTBrowser9 and FLVPlayer9. You also need a Flash browser plug-in, the one I tested with was version 7.
For YTBrowser9, after downloading and unpacking double click on it and it'll ask which browser you're using. Since I'm using Classilla, I clicked on Mozilla. Then, when you click through a couple of other boxes in Japanese and click cancel when it asks you to enter a Youtube url, Classilla should open up a welcome screen, again in Japanese. Find a link on that page that goes to an index file. Once you've found the index, you should see a red bar at the top with links to search, preferences, history, and a few others. Click on search, enter a search phrase, and then once you see a video you want to watch, copy its link, then navigate back to the index and click "?? URL" (its full url is "ytbb://www.youtube.com/dialog") and paste your link. Then you should see something like this:
To make YTBrowser9 easier to use, I added bookmarks for the search and enter URL links. I'm not sure if this is exactly how you're supposed to use it, but that happens when your Japanese is as rusty as mine, har.
As for FLVPlayer9, it apears to be a simple drag and drop application. Just take a saved FLV file and drop it over the app, then it'll open it in a web browser window.
One final note, I notice the most recent version of Classilla defaults to a site's mobile page by setting its user agent to a Nokia mobile device. Really cool move. Makes Youtube and other sites zip. Though you can change it back to "normal" on a site-by-site basis via cookies, or globally by changing the user agent.
UPDATE: It looks like YTBrowser9 is similar to MacTubes in that you have to downlaod a new version whenever Youtube changes something in their url protocol.