What kind of site do you have. I own a consumer review website and I have had great success with adbrite.com, they provide banners for your site and you get paid when people click on them. They also have several other options, like text ad link boxes that you can put on your site and companies pay a daily, monthly, or weekly fee to have a text link ad on your site. I am working on putting a text ad link box on my site now.
Google Adsense might work too, but I know a lot of people have had trouble with them, including myself.
Good Luck
The ConsumerTreehouse.com Team
Consumer Treehouse -- Your source for unsolicited product reviews