The loop libraries are all SampleTank specific, but they are really cool in that each preset contains the loop and all the components of the loop sliced up and spread across the keyboard, allowing you to play your loops in very unique ways (essentially turning every loop into a virtual drum kit, much like is done with Stylus RMX). This can produce some very interesting and useful results.
The Acoustic Guitar, Electric Guitar, Piano and Bass Collections contain stuff that I have been using for years, plus some more recently added stuff that I am pretty excited about. It is not all fantastic, but there are some great instruments contained in each pack... stuff I would not want to be without. I love this stuff, and at this dirt cheap pricing, I would not recommend passing it up.
Also if you join the GB via eSoundz, you'll get an extra bonus for each time you join (and you get to choose the different bonuses yourself).

http://www.esoundz.com/details.php?ProductID=4293" onclick="window.open(this.href);return false;