A Free Licensing System?

[C# Programming]

I am currently researching licensing systems for applications developed using C# targeting the .NET Framework v3.5. Are there any licensing systems that are 100% free? I don't plan on selling software on a large scale or anything like that and I'm not terribly worried about piracy. I'm simply looking to sell software (one particular application at the moment) to a small scale audience, priced around only about $15-$20. So I'm thinking that if there are no free systems I will just write my own. Because I'm not worried too much about piracy I figured writing my own may suffice. But I thought I'd ask in order to make it a little easier.

All I am really interested in is some algorithm to see if a user's key matches a "template" so to speak. If you could provide some links to related materials I can read them on my own. I am just having trouble finding anything that is useful for what I want to do.

Thanks,
Matt U.