The ways of getting a free stuff are quite simple, even more simpler that you can imaging. Doing this offline means that you can receive a freebie after you've made a purchase and there's a special bonus for it. Another way is giving you a small sample of a product "just to try". These are offline marketing methods, you've seen lots of them.
Doing the online marketing means that you are just a "man without a face", they cannot see your emotions about the product, so the only way to know what you think about the product is the participation in a customers survey, where you should answer to a lots of questions. Lot's of them. You know, that nowbody will do it without having something for it. Would you answer questions, type letters on your keyboard just for nothing? I think you would not. Therefor you'll be given a bonus, a free stuff for it. This is the perfect way to make a target group for this product as a part of a marketing research process.

To describe the process in a couple of words, i can say the following: you are participating and receiving a bonus. Let's the marketer's do their job without you.

Frankly speaking, i discovered some sites over the Web which are giving a list of such promo-actions, where you can receive a free stuff in exchange for participation. You can browse and see: what if the thing you want to have right now is available for free? So, happy freebying :)