The Wired article on Free has an excellent list of ways free can happen; the one I think we are all familiar with is advertising. We also are all aware of how traditional display advertising has lost favor to more accountable approaches such as contextual advertising of the Adsense/Overture model. But not all content lends itself to effective contextual advertising. Rouxbe has found a way to up the awareness and feeling of gratitude shown an advertiser by making them not the sponsor of the site but rather the sponsor of you using the site.
This is a very clever approach, in my opinion. I’d love to know how effective it is.