I've written a bit about Mactubes for OS X before. It's among the best kept secrets for PPC users as it allows smooth playback of Youtube videos even on something as low end as a Sawtooth. All you do is choose Quicktime for playback in the preferences, and Mactubes ignores the flash wrapper and plays back the videos as mp4 files. Much much easier on the CPU.
However, there's a chink in Mactubes's armor. Every once in a while, Youtube changes something in the urls Mactubes uses to retrieve the mp4's, causing Quicktime playback to go dead. You get a popup alert saying something like, "Can not open video. Please open URL with browser." Luckily this is a temporary problem as all it requires is a minor update from the developer, and everything's fixed. And luckier still, the developer is very prompt about providing updates for when this happens.
So anytime Mactubes gives you a popup alert like the above, wait a day or two, and by then the developer will likely have an updated version for you to download. And we can all safely return to wasting hours and hours watching random videos on the internet:)