Google AdSense is the leader in web-based content-sensitive marketing.
Webmasters place Google AdSense JavaScript code on their web pages and Google AdSense serves advertisements based upon the content of the web page.
Some webmasters are either not accepted into the Google AdSense program, or do not wish to do business with Google.
For those webmasters, there are many alternatives to Google AdSense:
Link: Tech-FAQ
There are CPM alternatives to the pay-per-click model that can offer video content publishers better monetization than Adsense. These impression based publishing networks have often been around for many years, while some are new to the field and thus have less stringent traffic requirements. While the CPM amount that can be earned varies depending on the niche and number of unique visitors/pageviews per month, for new media publishers this amount is often higher than Adsense revenue.
Link: ProBlogger