(Updated) Using free to build your audience sounds simple, but getting something to spread virally beyond your existing fan base is far from easy. Tweet For A Track offers a launching pad via a free online tool for artists to upload a track, tweet a link to their followers and encourage them to help spread to word.
To get the free track the recipient must repost your tweet to others before he is emailed a link to download the track. As his Twitter friends do the same your track spreads virally outward.
This graphic shows how the process works: