So this happened in the MacRumors Forum yesterday:
For a little background, user lotvai77 is the one who compiled Transmission 2.84 for Tiger PPC several months back, and CorePlayer is near legendary among powerpc users as the most cpu efficient video player (zen at PowerPC Liberation was one of the few who had a registered copy).
Now I can see what everyone was raving over. I previously found Mplayer from the shell to have the best playback, though for some 720p videos I had to disable the loop filter to avoid framedrops and this slightly degraded image quality. With CorePlayer this is no longer necessary and it has handled every 720p vid I can throw at it. I don't have any 1080p videos handy at the moment, but I've read from others CorePlayer can handle 1080p on faster Macs with ease.
Caveats: CorePlayer is the master of h.264 playback, but some mkv's I tried with it were wonky and avi's with AC3 audio didn't play any sound. Zen previously reported "little support for AC3 audio," so that's definitely been my experience!
One tip, if your cpu's too slow and you're still getting framedrops, go into Tools --> Settings --> Advanced and check "Disable AVC deblocking filter". I think this is the same as disabling the loop filter and it'll speed things up.
h/t Adam Albrec
Also: CorePlayer File Association Icons