You could use
TextLink Ads, Google AdSense or AdBrite.
With TLA, advertisers pay for link space on your site on a per period (eg, monthly) basis, so you get paid whether people click or not. You also get to approve which ads to allow.
With AdBrite, you also get to choose which Ads are shown. And then, you also have an option to show other Ad Programs in place of AdBrite Ads when the eCPM is higher for the program than on AdBrite.
Good luck!
