The new Mastodon album doesn't come out until next week, but you no longer have to wait to (legally) hear the entire album. The band has teamed up with iTunes to stream the album. You can access the stream here.
Note that you will need iTunes installed on your computer to hear the stream. If the above link doesn't work. Open iTunes, go to the Store in the sidebar then search for Mastodon. On their artist page, you will see a link for the full album stream.
I have not had the chance to hear the album in full as of this writing, but I am listening as I write this and so far so good.