Free gadgets...

When it comes down to it, that's all this is, advertising.

When you complete an offer, the company that you complete the offer with pays the referral site a fee, usually in the region of £50. So for each person you refer to complete an offer, the advertising company gets exposure to prospective customers, the referral site gets a fee, and you get one referal closer to your free gadget.

Let's look at an example from freebiejeebies:

Consumer (ie. you) completes 9 referals for a free iPod Touch, exposing the advertisers to 9 potential customers. For this service, the advertising company pays the referral site £50 ($99) for each referral, that's £450 ($895) in total. So, as a reward for your kind referrals, you get the free gift of your choice, in this case a free iPod Touch worth £200 (or if you're in the US, a slightly cheaper $300).

All in all, you're happy because you get a free iPod Touch, the referral site's happy because they make a profit and have a continuous stream of referrals, and the advertising company are happy because they get exposed to potential customers. Everyone's happy!