Advertising Banners
If your site has enough vistors you can easily use it as a source to generate revenue, helping you cover the cost of running a website or (with a bit of effort) making you some serious money.
Google AdSense allows you to display ads from their search engine that are specifically related to the content on your website. You receive a small sum every time someone clicks on a link or every one thousandth time the adverts appear. The ammount of money you receive correlates to the popularity of the pages the ads are displayed on and how close they are to the interest of your visitors.
If you want full control over the adverts you display how about using an advertising banner tool? This allows you to rent out space on your site and keep a track of the ammount of impressions and clicks that it generates. Unlike Adsense the adverts can be images or flash files but it is down to you to sell the space and invoice the clients.