Ishkur's Guide to Electronic Music is a big compilation of samples of every kind of electronica one could think of. I thought it was particularly interesting how well you can tell the difference between even slight variants of different styles with the samples he has, and his way of graphing it also illuminates a great deal (my favorites were Tribal and Darkcore, and I finally found the long sought Ayumi Hayasaki song I forgot I was looking for to boot).