Things you'll need: Jailbroken iDevice.
First you'll need to head over to cydia and add this source. --> Repo.HackYouriPhone.org
Next search for Linktunes via the search bar in cydia, then install the tweak.
Open up the iTunes app and you'll notice that at the bottom it has a LinkTunes tab. Click the tab and a search bar should show up, then just search any song you want to download. (via the linktunes search)
Click on the song then a play now and dowload button should show up, click the download button.
You should notice that the song has been added to your download tab. Once the songs has downloaded open up your Music libray and it should be there.
Enjoy your free songs!!!