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There is no way to sell a song for free in the online stores and streaming services, such as iTunes. If you see a song available for free download in iTunes, this is because the iTunes editorial team made that decision.
If you want to make your music available for free to the general public, you might try Bandcamp.
If you want to give your music away to certain, specific people you know -- such as radio stations, DJs, and friends -- an easy way to do this is to just email them your MP3s (or FLAC files, etc.).
You can also put your MP3s on a file hosting service and give whoever you want a link. For example, if you use Dropbox, just right-click on the file and select "Share..."