Signature tracking, track the stat of a signature, like how many accesses in one day.
Here is a simple proof of concept of how it can work even the signature are not hosted on your site(usually, dynamic signatures hosted somewhere else).
Here is the HTML code for the image below
<img src="/scripts/mgccl/sig.php" title="this script direct the HTTP header to somewhere else" />

and here is the code for the PHP below:
<code type="php">header("Location: http://www.lf2.net/forum/templates/subSilver/images/logo_phpBB.gif");

So, to from today on, you can just use the code above for the signature
From what you can see, you just have to add a few lines of code to do tracking, then half of my article's title is done.
Is there any point to this article?
Yes, it means you can show image from other sites and make people think the image is from your site(with some mod_rewrite). So you are doing a smart and evil hotlinking that most visitors would never know.