A couple years ago I did an interview with a guy who made $28,000 in one year doing this part time. It is work and requires organization skills to do it well, but it is legit.
I tried my hand at it using Project Payday and their training materials to see that it was legit.
Basically companies are willing to pay you to do trial offers for their products. They know that many people who do the trial offer will like their product and continue to be customers, so they are willing to pay people to sign up for trial offers.
So one of the main ways of making money freebie trading is signing up for offers like a Free Netflix trial, emusic.com, applying for a Discover Card, etc. The key to success seems to be having a calendar where you can keep track of when the trial periods end and make sure to cancel before they end.
If you are interested in learning more I definitely recommend Project Payday as they have a ton of training guides (free if you complete a couple offers) that will greatly simplify the process. Oh and they have a A+ rating with the BBB as well.