Linkbucks is a free service that pays web users for the links they share. They have sponsorship deals with top advertisers that allow them to display their ads when visitors are linked to a page.
For example, lets say you are trying to get people to sign up on your favorite ptc site. You are posting your referral link on forums and blogs, and so on. What linkbucks can do is make sure you earn something every time someone clicks on that link.