I came across this very goood and informative article on online advertising revenue streams and hence copied and paste the whole thing here for personal references
Google AdSense is the most popular contextual advertising network currently available on the internet. Many webmasters and bloggers earn money through this service. It’s available for anyone completely for free, AdSense is supported internationally – wherever you are!
But, unfortunately, many users simply can’t join this service – since Google don’t want to approve their requests. Also, many bloggers and webmasters are banned from Google AdSense, because they have broke Google’s terms. Once you get banned from Google AdSense – it’s very hard to re-enable your request!
Fortunately, there are many similar contextual advertising networks available for free. Of course, all of them allows you to earn money per click, like Google AdSense do.
6 Google AdSense Alternatives
1.) Clicksor – Clicksor is very good and quality Google AdSense alternative. Allowing you many advertisement types at good click-through-rate. Allowing you to use Contextual and many other ad types that are not supported by Google. (read review)
2.) AdBrite – One of the oldest and most popular advertising networks on the net. Advertising through AdBrite gives your visitors opportunity to buy advertisements on your sites easily. In other way, giving you related contextual ads to earn money through. (read review)
3.) Bidvertiser – Bidvertiser is professional contextual advertising network. Offers you only contextual and feed advertisements. Very similar to Google AdSense. Giving you opportunity to earn money by displaying related textual ads on your site. (read review)
4.) Chitika – Chitika is founded in 2003. It is the leading blog advertising company with a network of thousands of blogs and websites and hundreds of name brand advertisers and merchants. Offers you to earn money by using many ad techniques and options. (read review)
5.) Text Link Ads – This service is not actually contextual advertising service. TLA allows you to earn money by selling text and inline text advertisements on your site or blog. Choose price and sell textual advertisements on your site with this service. (read review)
6.) Yahoo! Publisher Network – Yahoo are currently playing catch up to Google and are trying to release their own context sensitive advertising service. This promises to be an excellent alternative to Google Adsense but in reality is likely to be a simple copy with the same restrictive terms and conditions as Google, including $100 minimums on payouts. Right now it’s not available internationally.