I did some research back then and saved some bookmarks. I haven't tried any of these. Let us know which one worked for you or anyone who used one can post some feedback.  I wish DNN software creates some kind of a licensing api module developers can hook to. I am developing a module and the I have deferred the licensing stuff to the very end. It's the least desirable part and the one that contributes nothing to the feature set. 

http://www.activelocksoftware.com

http://www.eziriz.com/intellilock.htm

http://www.ssware.com/cryptolicensing/features_net.htm

http://www.licensingdotnet.com/pricing.htm

https://github.com/ayende/rhino-licensing

https://installkey.codeplex.com/

https://softwareprotector.codeplex.com/

http://www.codeproject.com/Articles/35009/How-to-Generate-and-Validate-CD-Keys-for-your-Soft

http://www.developer.com/net/csharp/article.php/3074001/Applications-Licensing-using-the-NET-Framework.htm