Where is a good collection of freely licensed instrument .wav samples?

I require full samples for a variety of instruments. One such site that provides this resource (http://theremin.music.uiowa.edu/MIS.html) is good, but there aren't enough non-classical instruments (e.g. steel guitar, drums).

I need a different resource that provides instrument samples such as ones the MIDI specification supports.

Side note: I only require middle C samples for each instrument.