Traffic Exchanges for Free Traffic
Traffic exchange sites (also known as hit exchanges) offer both free and cheap advertising. These advertising sites provide traffic by having members exchange page views at certain ratios. For every site you visit, you will earn a fraction of a credit (or a whole, depending on the TE) that can be converted into a page view to any of your web pages.
Traffic Exchanges come in different exchange ratios. Typically, 4:1, 3:1, 2:1, and 1:1. For example, a ratio of 2:1 would require surfing 2 pages to earn 1 visit in return. 1:1 would require 1 for 1. Higher ratio TEs tend to give less bonuses but tend to be more expensive to upgrade than lower ratio TEs.
To be honest, I believe this is the cheapest form of advertising. To my experience, traffic exchanges are good for fast page views, exposure and maybe branding. However, not for selling or getting sign ups. Nonetheless, like any form of advertising, they can be effective if used properly. There really are marketers out there that succeed with traffic exchanges.