When developing a website, I really like to use Google’s Adsense. Not only can I customize the ad units to integrate into my website, but I can also use graphical banner advertising, which not only generates revenue, but it helps give the appearance that advertisers want to be advertising on a particular site. Technically, they may not know they are advertising on certain websites at the time, but they are able to block them if they don’t wish to advertise there any longer.
One drawback though is that certain advertisers, who may have opposite beliefs than your website, can still advertise on it. For example, on a religious website, a website that is owned by a group who has conflicting beliefs can still place advertisements depending on the keyword match they are choosing. These links can take visitors of your website to another site, potentially angering the visitors, especially because people don’t always realize how Adsense works.
A website owner is able to block certain urls from appearing, however, there may be hundreds of different website urls with the same message that want to advertise, so it can be a never ending battle. That said, it’s very important to monitor your websites to make sure objectionable advertising isn’t being displayed. While the ads don’t violate Google’s TOS, it could violtate your own site’s standards.