Money. A tricky subject to bring up. It's been proven that merely activating the concept will induce selfishness. But bear with me here. This is a public service announcement, not a get rich quick scheme.
If you have a website that gets a reasonable amount of traffic, you've probably considered adding advertising at one point or another. Maybe you're dreaming of quiting your day job to work full time on the site or maybe you just want to cover some basic costs.