You may be looking for alternatives to Google Adsense for a variety of reasons. Your account may have been closed for violation of the TOS, or you may want an alternative to Adsense for an adult oriented or other type of site that may not qualify.
The biggest reason many publishers want Adsense alternatives is to further monetize their website. With most programs such as Adsense you are allowed up to three pay per click type ads on a page. This means that you can diversify with two other pay per click ad groups on your site.
Here is what Adbrite has to say on it's website about using both Adsense and Adbrite.
"AdBrite has no policy against using multiple ad networks on the same site. Many of our clients do use our service in conjunction with AdSense and other ad networks; though, we recommend you contact those networks directly for their specific policies regarding multiple networks."

Here is what Widgetbucks has to say in the FAQ section on their website about using both Widgetbucks and Adsense. "Can I run WidgetBucks with Google AdSense?

Yes, WidgetBucks is compatible with all other ad systems."
Placement of paid advertising on your site is one key element to success. You will want to place the higher paying advertisers closer to the top and bottom of the page. Ads placed along the side are not clicked on as much.
Which Adsense alternatives you choose and where you place them is up to you. You will want to do a trial and error experiment, by placing Adsense ads in one spot and alternatives in another, and switching places.
Two of the most popular Adsense alternatives are Widgetbucks and Adbrite. Both have been around for a while now. Widgetbucks was founded by one of the former e-Bay management.
Widgetbucks is another good alternative. You have undoubtedly seen their ads from merchants like Best Buy, e-Bay and other major retailers. In addition they often feature catchy yet slightly annoying (but money making I should add) ads that say something "You are the 10,000th Visitor. Click Here to Claim Your Prize". The good thing at that these ads are shown only a fraction of the time.
You can find out more about these two good Adsense alternatives and several more here: http://linkbee.com/37009