They are often real, but you usually have to subscribe up to SOMETHING. The idea is that it's target-advertising - instead of wasting money on standard ways like magazine postings, companies pool some of their budget into these sites. They give you free stuff if you and a few friends sign up for their services. Like pyramid marketing, it's only worthwhile if (a) you find an honest company, (b) the products interest you, (c) you are near the top of the pyramid, and actually have friends who are also interested.
If you can find offers that are cheap and worthwhile, you can usually get the free product much cheaper than buying it outright. The sponsor sites are the ones paying the most, and they are achieving more product awareness than through standard ways of advertising. They only need a small percentage of the trialists to actually want to continue with a product or service, and often there are people who are genuinely interested in doing so.
As long as the site is not a scam, these can be good offers. Still, NO TY.