Adbrite, is currently one of the best alternatives there is to Google's adsense. While they do not offer the same large selection of ad formats that Clicksor and Google Adsense provide you they do offer the most commonly used ones. In addition they offer inline page links with have some great click through ratios as well as interstitial full page ads which offer an excellent way to monetize all traffic to your site not just traffic that clicks on your ads. Their payouts are also very competitive. They have more relaxed terms and conditions than Adsense and are much more accepting of smaller publishers including bloggers.
If you're a publisher, use AdBrite to set your own ad rates, and approve or reject every ad that's purchased for your site or just have AdBrite auto accept ever ad. AdBrite enables you to instantly sell ads to your visitors via a "Your Ad Here" link, in addition to selling through AdBrite's marketplace and sales team.
Revenue is typically split 75/25 in your favor. Through a small snippet of HTML placed on your site, they handle serving, scheduling, billing, customer service, and sales. About half of AdBrite's sales are generated from the marketplace and sales team, while the other half are generated from users clicking "Your Ad Here" on your website.
While AdBrite can provide publishers with more revenue and better ads than traditional ad networks such as Google AdSense, they work fine along-side them as a way for you to generate additional ad revenue by selling ads directly to your visitors -- something the other ad systems don't do. Ad Brite also lets you select your own minimum bid prices and give you the option of showing an alternative ad service such as Clicksor when bid prices fall below your minimum.
If you're using AdBrite on your website you also have the option to turn off AdBrite's "run-of-network ads" and AdBrite will only display ads that have been approved by you allowing you to prevent competitors ads being shown on your site. If you have no ads running, AdBrite will display nothing but "Your Ad Here" or your alternative ad provider.
Note, from my experiences it can take a day or so from when you signup with AdBrite and put their code on your website to actually start seeing relevant ads showing up. So if you see the message "Advertize on this site" just be patient and give it a day or so and you should start seeing relevant ads showing up.
AdBrite also accepts Blogs and Bloggers as sites so if you are fed up with other programs turning you down because their terms and conditions restrict Blogs try AdBrite.