Including ads on your Web site can be a great way to defray maintenance costs, and these days they’re relatively easy to come by. There are four basic forms of Web advertising: click/impression-based ads in which the advertiser pays you via a third-party site such as Google or AdBrite; third-party sponsorships, which are generally only available to larger sites that generate a lot of traffic; affiliate programs that pay you a percentage on sales generated from your site; and ads that you secure yourself (such as for local or regional businesses).
Whether you decide to place third-party ads on your site will likely depend on your willingness to surrender space on your site. Assuming you can effectively integrate the ads into the page design without detracting from your users’ experience, you should run a test to make sure cannibalization (which is when your site’s traffic or sales stagnate or drop while traffic to the ads increases) doesn’t occur. If it does, your options are to redesign your site, move the ad placement, or improve your product offering.
For more on Web ads, be sure to read How to Monitor the Effectiveness of Online Advertising.