Google AdSense

If you have a website I strongly suggest you sign up for Google's AdSense program. It's definitely one of the best options out there.

Pros:
- free (no fees)
- reputable company
- transfers to you checking account (in several countries)
- targeted ads (the ads will match the content of your site)

Cons:
- minimum payout is $100 (could take a while to reach for a new site)

To sign up for AdSense use the button in the sidebar (to the right and close to the top).

For more information see Wikipedia