The classic.
advertisement via banner has for several years proved to be very successful.
By inserting the bannercode into his site the user is able to show advertisement of other users on his site.
In return his own banners are shown in a certain ratio on websites of other users.
The bannerexchange is a very effective and not much time-consuming kind of advertisement.
The user just needs a grafic (gif, jpg, etc) that can be shown on websites.