To comply with the terms of the user license, you are required to buy additional licenses for each domain you use it on. I'm happy to say that people are doing so. However, there are a couple of things I can say to clarify my position:
* There is no serial code, encryption or other anti-piracy measure in simpleContact Pro. I prefer to trust my users and spend my time improving the product rather than developing such mechanisms (which a determined pirate could remove anyway).
* Although I don't actively prevent a user from buying a license of 'Pro and installing it on multiple domains, it does breach the license and I do not support unlicensed domains or users.
* I plan to offer a "developer" license option in 2009. You will be able to buy this license to use 'Pro on as many of your own websites as you wish. See the volume licensing thread for more information.