UPDATE 2: FlashVideoReplacer has ceased development and the developer has removed it from Firefox's add-on site. I subsequently wrote about another flash alternative, quvi, here, and some cross platform alternatives here.
Given that the Youtube player Minitube is somewhat broken in Debian Wheezy (about half the videos don't load), Linux PowerPC users may be interested to know there are some alternatives to Flash so you too won't be deprived of watching time-wasting Youtube videos day and night (this is not a confession).
Probably the best way is with the Iceweasel add-on FlashVideoReplacer. I've written about this add-on before for OS X, but the Linux version allows you to pass on videos to a standalone player, no flash or Quicktime plug-ins necessary. So you can have them stream in gnome-mplayer or xine or whatever, and you got Youtube videos on your desktop. Or if your CPU's too slow to be downloading and playing simultaneously, you can just use FlashVideoReplacer's download option and play the video afterward.
There's just one problem with that, though. FlashVideoReplacer only works on Mozilla browsers, so if you're using Midori (webkit) you're out of luck. Or are you? Turns out Midori can use some Greasemonkey scripts, one of which is Viewtube. You have to enable user add-ons in the extensions section of the preferences, then at userscripts.org go to the Viewtube page and click the install button. The next time you visit Youtube you'll see non-flash playback options just above the video frame.
There's also html 5, but on PowerPC I don't think you get any hardware acceleration so you probably need a G5 to get something playable. Also, a flash section at Ubuntu's PowerPC FAQ talks about Gnash and Lightspark and a few other suggestions like spoofing your browser's user agent to a mobile device.
h/t to the users at the MintPPC forums for providing much of this information.