What is the purpose of the following image, found on a page in the hypothetical site www.maindomain.com?
<img src="http://www.anotherdomain.com/image.gif?track=6bb3f01axu7Y&aid=00995"
width=1 height=1 border=0>

Any takes?

they are called web bugs and are use as hit counters or for some other purposes:
http://www.outpostfirewall.com/guide/faq/web_bugs.htm
http://www.adsubtract.com/advisor/feat_bugs.html

that "image" is probably not a real image, but rather a script that creates an image on the fly. It then performs tracking everytime the image is loaded. This is an alternative to javascript code and is usually used by affiliates to track sales.

You beat me, danburzo. Good links.

second link is down though. it redirects you to the home page.. funny, seemed to work when i posted it. anyway you can find loads of info, just google for "web bugs"