My personal favorite is the mobile YouTube site, but on G3 Macs and slow G4 Macs you are limited to using TenFourFox for that and TenFourFox is just too heavy for that. For those machines, I use the web version of PPC Media Center 7.x, as it gets the job done quickly enough for me and is able to work on any Power Mac running Mac OS 9 or later. It may even work on Mac OS 8, but I haven't tried that yet.
SMTube is another option works on Leopard and Linux, and is able to provide full res videos at a decent framerate on faster G4s and all G5s, but G3s are (mostly) left out and even then, it doesn't run well on them.
For the fastest Power Macs, the desktop YouTube site is still usable. If you have an 867MHz G4 or better with 512MB of ram or more, you can use the desktop YouTube site on Leopard-Webkit. To use Leopard-Webkit, you must be running Mac OS X 10.5 Leopard. For the absolute fastest G4s (1.5GHz and 1.67GHz are the fastest that shipped with Macs, it is possible to upgrade some to a 1.7GHz one and OC to 2GHz.) and all G5s, you can use the desktop site in TenFourFox. TenFourFox will allow you to go to a higher resolution and fullscreen, but your FPS will likely drop and the whole experience will be worse in general.
Overall, the slowest PowerPC Macs I've found to be able to use YouTube are the 233MHz iMac G3s, and that was at 240P using the web version of PPC Media Center 7.x. The desktop YouTube site is likely unusable on your PPC Mac, and if you have a pre-G3 Mac YouTube in general is going to be unusable. The solutions I listed here aren't going to run well at all, and most won't work as they all (except for PPC Media Center 7.x) require Mac OS X 10.4 or greater.
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