Right in the middle of our upgrade, while reading through Justin Palmers blog (based on the up and coming RoR Mephisto by the way), I came across a service called Text Link Ads.
http://www.text-link-ads.com/
Similar to google adwords you can add sponsored links to your site - the thing is, they are less obtrusive meaning they slot in nicely with your design and navigation an probably get you a better click-through rate.