Sometimes this is worth it if they can specifically target the user you advertise to. For example, Ask Jeeves , where users can ask the robot a specific question, can advertise when a question relating to your site is asked. This sort of targeting can be invaluable to a webmaster. It's a good idea to find out how your ad will be marketed, otherwise you could end up displaying on a Gardening page. Text is usually all you really need.
Many sites now offer advertising in the form of a text link.
This text link usually looks like a standard link, and can reside anywhere on the page, whether that be in the top 'headline' spot, or on a side panel. Text links can be effective so long as they are written well. Be short, concise, and to the point. Let the potential visitors know what they can expect by clicking that link in the least amount of words necessary. When your traveling around the web, pay close attention to any banners that catch your attention.