What D-Sane is talking about are Akai-formated sample cds...
They're not in the usual ISO9660 format, and are only readable by special programs on the computer. However, a few good Akai sample cds would be a great investment for you to build up a sound library.
Also consider the expansions IK Multimedia puts out for SampleTank. I use their strings expansion all the time. Check out Sonik Synth 2 as well. It has about 5000 patches on 2 DVDs that you can load up in SampleTank and you get a discount for being a SampleTank owner.