Not free, but from some countries (I know US and Canada are two), you can get a boomerang pass, which will allow you a certain number of flights for a price.
Whether it is a good value, I don't know. I did as Globaliser instructed, and even though my flight to Australia was on QF (much better than the Untied alternative), I did domestic legs on JetStar (nobody looks good in Orange Polyester) Virgin Blue, and Qantas. All of them cheap if not reasonable.