There's an offer available from the iTunes site until 28 September - send a text message (which for most people is at their standard rate, ie. costing roughly 10p) and you receive a voucher code for a free track from online music store iTunes, which normally cost 79p or 99p.
How? Text one of the following words - ROCK, POP, LIVE, or ITUNES to 85100, and you'll get a message back with a code. Log into the iTunes store, go to buy a music track and you'll have the option to enter a discount code. Pop the code you were sent in there, and the track will be free. You have until September 30 to use the voucher.
You'll need iTunes music player installed on your computer in order to purchase from iTunes store, so if you haven't got it, download it!
Anything else? Apparently you can do this multiple times, and receive multiple codes. While this may be possible, its important you're aware it's against the terms and conditions of the offer - you can read them in full at http://www.apple.com/uk/itunes/sms/