Why are there so many stupid giveaway ads on the internet?

[bobertdude]

You know those stupid ads claiming to give away free stuff? Or the ones saying they need "product testers" and other nonsense? Why are these ads so common? How did they get on so many big websites (i.e. Yahoo, Myspace, etc.)? Are they legally even allowed to use other products (Mac computers, video games) in their advertisements? Do they actually work as advertised?

I know this is a lot to ask, but I've always wondered and have never been able to get a good answer.