Hello,

You can put the banners or other...and the necessary Javascript code (to detect), in your own site... and let the users copy it, and place it in his Site.
Try to visit this site (it is in portuguese)- and in "Connect with us" page, there is examples of what I'm saying...it's easy...and very simple, and FREE of charge:

http://www.truquesedicas.com/liga_te/index.php

The Javascript code associated to the image, is like this:

<center><a href="http://www.YoursiteName.com"><img src="http://www.YoursiteName.com/imagens/ImageName.gif" width="120" height="89"
alt="Your Site name- A short message" border="0"></a>

Note: They should copy this code and save the image, and place it in their Site...ok ???
Hope I helped you.

Amit