Not too many years ago I came across a web service called StumbleUpon. They are in essence one of the first social bookmarking sites on the web, before the term “social bookmarking” was ever coined.
In layman's terms, social bookmarking is a site that allows communities of people to submit their favorite web site links for others to browse and vote upon. If you've read anything in the last four years you've probably noticed the trend of user-driven content sites like StumbleUpon, Digg, Youtube, and so forth. The premise is that the control of these sites' moderation is given back to the very community that supplies the site with content.
Traditional spammers have a tough time penetrating the power of thousands of real human reviewers so the only real tactic to gaining entry to these valuable traffic streams is through submission of quality sites. Odd concept right? Build something you yourself would bookmark and put it in front of the eyes of thousands (if not millions) of similar minded people and you've got a big spike of traffic.
It's a concept called parallel marketing – which is fancy talk for FIRST identifying the people who buy hoodia, get mortgages, or collect figurines and THEN finding something else they also enjoy. A great tool to figure all this out (aside from your common sense) is a unique web service called Quantcast. Quantcast is a glorified demographics site which identifies similar interests based on keywords and site names.
Advertisements, Adsense laden sites, sites with pop ups or installs. Again, this is another instance of the need to use latent advertising. With just a few clicks your junk submitted site can be knocked down the ranks by other users who dislike spam and resent you for trying.