Earn with PPC Ads(Pay Per Click).

Google Adsense is the most popular and widely used option.
They serve contextual ads (image or text) based on your
content, and pay when someone clicks on the ad. How much
you make on Adsense depends on several things.
First,
advertisers bid on keywords, so if you happen to write about
a topic that has high paying keywords, then each click will
be worth a lot more than another topic with low valued
keywords (keywords that has really low bids).
Also, people
who came to your site from search engines are more likely to
click on these ads than people who are repeat visitors.
It
makes sense then to spread out your Adsense ads to provide
easy access for search visitors.
Other PPC options include Bidvertiser and AdToll .