We used a number of them Gary.
Castanet was one of the most challenging. No codec at almost any bitrate is transparent to it. Unfortunately I can't find the MPEG version online anywhere. This one is close to give you an idea:
http://www.freesound.org/samplesViewSingle.php?id=57299
You can actually hear the pre-echo in the streaming version.
Next was Harpsichord. Another MPEG clip, it has rich harmonics that would magically change.
Then we had simple things like people applauding. This was an MPEG test but there wasn't anything special about it.
Male and Female solo voices are also commonly used. Suzanne Vega's song "Tom's Diner" is the track where she just sings with no music.
All in all, the theme was to have a a focused test clip that would severely push the codec. No audiophile tracks. No classical music since it is harmonic and pretty easy to compress.
In the old days, I used to have my own collection of popular music I used. Unfortunately I lost one of my servers and all the files are gone.