How about AdSense?

In terms of earning money on Adsense, your mileage varies. One website with the 10,000 uniques a day can earn $50 a month while another may earn $5,000. It is not easy to predict how much you will earn from Adsense. The only way you can learn about how your site will perform with Adsense is through trying it.

The amount you can earn will depend on the

1. Responsiveness of audience to the ads = Users looking to book a travel to Spain comes to your site about tourist destinations in Spain is a site that will do well with Adsense as users are more likely to click on an ad about hotels in Spain. However, if you are a gaming website where the main purpose of the user is to play games on your site, then Adsense will not perform as well.

2. Ad format = some types of ads do better than others depending on your content and layout. In our case, large rectangles in the middle of the content is the best, while leaderboards do not generate as much as income. Skys are the worst for us. Experiment and measure the results via channels and see which formats work best for you.

3. Ad placement - check Google's heat map as they have tested where the best placements are https://www.google.com/support/adsense/bin/answer.py?answer=17954&ctx=en:search&query=adsense+heat+map&topic=0&type=f

4. Ad colors - sometimes ads blended into the content works wonders, but sometimes ads that contrast your site colors work best

5. Number of ad units on a page = we are allowed maximum of 3 ads + 1 ad links + 1 search box on a page. Maximize the allowed number based on the resulting look of your page (you don't want an overkill of ads). Users going to your page and reading your content may ignore the banner or rectangle at the top of the page, but may click on the ad at the bottom of the article

6. Smartpricing - the big unknown in Adsense. No one knows how this actually works

Experiment with the factors above (except smartpricing, which you can't control), and see which combination works best. Remember though that not all sites do well with Adsense - even if you get gazillions of traffic but your visitors are not interested in looking for ways to spend their money, they won't be interested in your ads and won't click.

I suggest you read the following articles:

Google Adsense: How Small Business Entrepreneurs can Earn from Online Advertising http://www.powerhomebiz.com/vol123/adsense.htm
Earning Revenues from Contextual Advertising http://www.powerhomebiz.com/vol139/contextual.htm
8 Tips for Maximizing Contextual Advertising Revenues http://www.powerhomebiz.com/vol146/contextual.htm
Contextual Advertising: Why Are You Earning Only Pennies? http://www.powerhomebiz.com/022006/adsense.htm