Fastclick is a revenue stream that will be suitable for some highly trafficked blogs. We trailed it recently on our Olympics blog and found that with the levels of traffic that we generated at the height of the games that it out performed all other revenue streams on the site including Google.
They offer a range of ads – mainly paying on a per impression basis. Pay outs depend on the ad size, style and content but are generally very very small. As a result a blog with only a small amount of visitors would probably find it more hassle than its worth to register and run with.