An oldie but a goldie, this is the link 9 (direct to Amazon) to approximately 890 free mp3's that Amazon give out.
I like my metal, so I don't recognise many non-metal bands.
I can see on the list three big band names at least: flight of the conchords, sepultura, the datsuns - so it isn't rubbish at least!
Lots to explore!
I did a search and couldn't find this mentioned before, hopefully it isn't a double up.
People didn't seem to notice that this is for Amazon *US* (as it was tagged) and are having trouble seeing them if using the UK store. This link does work and is bookmarked by hundreds on Delicious.com. The US store is worthwhile signing up to, you get more deals than the UK one on almost everything.