There are some great deals at Amazon right now:
Order this free game (you don't have to actually download it), and right now get a $5 credit towards MP3 in your e-mail (it doesn't say how long that will last - I did it Friday and got my credit). You have to use the Amazon game downloading software.
Many free world music samplers are now available here. Again, you have to use the Amazon music downloader, but I like it because it transfers the tracks directly to my iTunes - no importing.