Amazon recently changed over to a new set of 100 albums @ $5 each--typically, they are a set of old and new albums of many genres (no classical this month, for some reason, but the other genres appear to be represented).
See them all here.
Or, if you want to see them by category, go here and click the links in the "Department" section of the left sidebar--each link will show only albums of that category.

This is what the "Department" sidebar looks like, btw, except that the entries are links on the actual page (the numbers next to the category names represent the number of albums per category).

Department MP3 Downloads    MP3 Special Deals    * Alternative Rock (18)    * Blues (4)    * Children's Music (2)    * Christian & Gospel (3)    * Classic Rock (3)    * Country (6)    * Dance & DJ (7)    * Folk (2)    * Hard Rock & Metal (5)    * International (5)    * Jazz (9)    * Latin Music (1)    * Miscellaneous (3)    * Pop (23)    * R&B (11)    * Rap & Hip-Hop (11)    * Rock (27)    * Soundtracks (4)