There can be many reasons why you do not have Adsense on your site. One of the most common ones is that you have violated some Adsense policies and Google has banned your site from further participating in the program. Another reason could be that your site is new or does not qualifies for Adsense because of it does not contain unique content or the kind or the language of the content is not allowed. More simply you may be not happy with Adsense earnings and are trying to find out other programs which can either replace or complement Adsense and can help you in making more money from you site.
Some of the most popular alternatives for Adsense are probably Chitika, InfoLinks, AdMaya and Kontera. While Chitika shows relevant ads for visitors who come to the site searching for a particular keyword, AdMaya allows you to place as many ads as possible anywhere on your site.
Infolinks and Kontera show text based ads within the content of the site. They will analyze your site and find out some keywords from a page and make them hyper link ads. These programs can work in parallel with Adsense and you can use both Adsense and any of these two programs at the same time on your site.
Some of the advantages of these alternatives are that it is easy to get your site approved under these programs. They have a low minimum payout and most of them also pay through paypal. Although they might not even make 10% of what Adsense will, they are still useful in their own ways. The best way to use them is to make them work in integration with Adsense. If they harmonize properly with Adsense, they can easily generate some extra cash without harming your site.
The bottom line is that even if you find an alternative for Adsense, you are most likely to earn less from it than what you would with Adsense. Therefore your best bet will be to stick to Adsense if possible and implement other ad sharing programs to run in parallel to it. In this way you will be able to use both Adsense and other programs to maximize your earnings. However, if your site just do not qualifies for Adsense, then the above solutions can work as valid alternatives.