If you ask most people, especially newcomers to internet marketing, which contextual ad service they use on their website, it’s fair to assume that most people will say Google Adsense. In reality, it is probably the easiest way to make money from a website - nothing is required of the site visitor other than a click, from which you can make anything from a couple of cents to a couple of dollars.
There are other services out there, such as Yahoo (USA only), Adbrite and DClickAds which offer contextual advertising services but there’s no question that Google dominates the market. And therein lies the problem. It is too easy to become dependant on Google for revenue, which can be disaterous if your account gets suspended (- something that seems to be happening a lot) with little or no chance of a reversal. Live by the Google, Die by the Google. I personally don’t rely on the money that is currently in my adsense account and try to mix my revenue sources as much as possible, in fact most of the income from my sites comes from affiliate links.
Although Google does dominate the market at present, that isn’t to say you can’t make money from networks such as Adbrite and DClickAds, and the best way to find out what works best is by experimenting. For example, David runs Yahoo on one of his websites and he tried out the Adbrite interstitial ad service one day, only to discover that his daily revenue shot up. Basically, there are tons of networks out there: Text Link Ads, Adbrite, DClickAds, Advolcano and many more which can all potentially provide an additional revenue source, so you don’t need to rely on Adsense for income. John Chow has a great list of advertising networks which can be found here.