Xbench: Comprehensive Macintosh Benchmarking
Free and very nice. I found my iBook had an xBench score of about 35.1. After running the test I clicked submit, and was able to view my results in context. They turned out to be quite typical -- so my slow performance at times is probably related to application issues rather than hardware or global OS issues.
By comparison the G4 iBook scores about 2.5x as fast as my 600 MHz G3. A nice gain!
The original G5 is about 5.5 times as fast at 175. There are no scores on the G5 iMac, but I'd guess it's only 30% faster than the high end G4 -- say 120. So 3-4 times as fast as my G3 iBook.