I know a lot more about you than you think! I know that about 62% of you are male, 39% of you are in the age range 35 to 49 and 81% of you have no kids in the range 0-17. Contrary to what many of you lead us to believe(!) 36% of you earn less than $30k a year. Approximately 45% of you didn't go to college. I also know about a lot of other sites that you are interested and therefore what affiliate offers to promote to you!
How do I know all this?
warriorforum.com - Quantcast Audience Profile
Now, the above example is a just a bit of fun. BUT, imagine the power of this information.
For example, if you are interested in promoting a popular Clickbank product, go onto quantcast, enter the address of the sales page and bingo(!) you know everything about your target audience and therefore exactly how to market to them.
And did you also know that there are some PPC schemes that enable you to target a particular audience profile?
And best of all, this information is all available for free!