Google Adsense is great, but its not the perfect program for publishers wishing to get in on some of the PPC action. The reason it isn't perfect is primarily the fact that Google cannot perfectly target ads to your visitors, only to your content. Your visitor's desires and your site's content do not always match perfectly.
There are other similar programs though, two of which are SearchFeed and RevenuePilot. These programs both allow you to specify the keywords that you wish to advertise or link to and thus you can achieve greater targeting than you can with Adsense. This of course assumes you know your visitors well enough to target ads to their desires.
SearchFeed and RevenuePilot both allow you to simply link to the search results or to include a search form on your site. This allows you to do in context advertising - which is another thing Google cannot do. So instead of taking up valuable banner space with these ads, you can simply use text links within your content. For instance in an article about web hosting you could link the word "web hosting" to a result set. When implemented in this manner these programs are creating additional income. In other words you don't need to take up banner impressions from another program so any income gained from these programs can be in addition to what you already make.
In addition to the search result method of integration listed above (that allows you to simply place text links throughout your content), both programs also allow you to create creatives like Google Adsense where the ads are listed directly on one of your content pages without the user having to first click to a set of search results. Both integration methods have their benefits and they are best implemented in different ways.
Both services have referral programs as well. SearchFeed grants you 2-5% revenue share for publishers you refer and 5-7% of the initial deposit for advertisers you refer. RevenuePilot offers a 10% referral fee for publishers. Though I should warn you that you should not expect to make large amounts of money with referral fees. Referral programs like this are great for additional small amounts of income, but most people who go into such programs hoping for huge gains usually end up disappointed.
All in all I have to recommend both RevenuePilot and SearchFeed. The programs are an excellent way to create new revenue streams for your site. You're not going to find the same CPC rates you enjoy with Adsense, and you will likely not make more than a hundred dollars a month unless you have traffic numbers in the high six figures atleast. However, since these revenue streams can be in addition to whatever you make now, there is very little reason to not implement them. As for which program to use, well you should check CPC rates for your keywords at both programs. You can check SearchFeed's from their homepage, but you'll need to sign up with RevenuePilot to check theirs. Then, go with whichever one is going to make you more money. You might also be able to integrate both programs by combining their XML feeds into one search result that ranks the highest paying listing first, regardless of which company it is from.