Although i've only spoken about displaying ads on your site that will earn your revenue another great idea is to place ads/links on your page(s) as a link exchange agreement! There are plenty of sites that provide exactly this type of service such as, and

These types of link exchange program will require you to display a link somewhere (anywhere visible) on your site and in exchange they will list your site and its description in their visitor accessible database. It really can be a great way to get some free advertising, and if you're using Google Analytics you can track exactly where referrals to your site have come from, meaning you can check which programs are working for you and which aren't.