The are several modules that support advertising on your site.

• Advertisement module - http://drupal.org/project/ad)
• BannerConnect Adspace - http://drupal.org/project/bannerconnect_adspace
• Classified Ads - http://drupal.org/project/ed_classified
• Curlypage - http://drupal.org/project/curlypage

You can also create your own ad blocks for advertising your own nodes. Using views, you can create blocks that show nodes you want to highlight.