Free banner exchanges are a good way to promote, or advertise your website.  You submit a banner (meeting their qualifications) for them to display on a website (preferably targeted to a similar website), and they display a banner on your website at a certain ratio (e.g. a 4:3 ratio means your banner is displayed 3 times for every 4 times you display theirs).  Some services offer "Free Credits" (free banner displays) for joining.