Quite a lot of bands seem to be doing this.. releasing a song for free download for a week before they distribute it comercially. I've got at least 4 free songs to date. Not a huge fan of any of them, but hey... a free song is a free song... and everyone likes a freebie now and again.
Current free song: Coldplay - Violet Hill. Available for download for the next 6 days I believe. It's a pretty good song actually. You have to enter an e-mail (I used one of my many ones to redirect junk mail to), and a post/zip code, for which I entered a fake one (they sure ain't knowing where I live

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coldplay (then click on the black 'song' tab)
Otherwise, it's like a legal version P2P networking if you think about it.
I'll keep this updated with any other ear-tingling bargains I find.
Add your own also If anyone happens to come across any. (making sure it's nice and legal)