How to track who is using my plugins?

Hi,

I was wondering if anyone could give my guidance on how to track my plugins?
I have a few that I'm working on putting to the marketplace in the next few weeks but want to track which website is actually using them.

The reason why I'm looking into this is to prevent someone from buying the plugin for a specific website but also using it on other websites not specified when buying the plugin.

Either using a centralized tracking code inside the plugin framework or something that would work similarly?

Any help would be greatly appreciated!

At nopTools.com, our plugins don't 'track', but rather look to see that the URL that the site/plugin is running under matches the URL in a license file...

When a customer buys one of our plugins, they must enter their URL (or domain), and then when the order is paid, we generate a license file that they can download (My Account > Downloadable products), and put it in their plugin folder.  

It requires that you modify your site to generate a license (handle the Order Paid event), and your plugin needs to read the license file, and compare it to the site's URL.

(A while back, there was a post discussing this, and I think someone may have provided sample code)