Google Adsense wins hands down when you need ads for the content you publish. However, it's not the only one out there. Amongst others, Adbrite and Chitika are quite popular. I have been trying Adbrite for quite some time but I guess the site's traffic is too low to generate any value at all. Chitika on the other hand had its registrations closed for some time, and just recently my application was approved.
Chitika positions itself as second only to Google Adsense. It has a Premium Pay Per Click (PPC) program, and is said to be compatible wit Google Adsense (i.e., you can show Google Adsense and Chitika ads on the same page without violating any terms and conditions of the either two).
Chitika | Premium capitalizes on "search engine" traffic (currently only from the "developed" countries). The idea is that when somebody uses a search engine to reach your site, he might be interested in knowing more about the "keywords" he searched for. Chitika ads on your site, in that case, will show ads relevant to the search keywords. It's kind of Google Adwords for advertisers but ads appear on a website chosen from the search results instead of the listing given by the search engine.
I believe Chitika is much more revenue earning when compared to Adbrite. Do give it a try. It's surely far behind Google Adsense but still a worthwhile alternative.
There is a referral program as well by which you earn more for referring publishers to Chitika. The links above contain my referal id. Sans referral link is also given: Chitika.