What Web Part Licensing solution works with SharePoint ?

I am a SharePoint Developer and I have developed a couple of web parts that I would like to license to clients. I want to license this to clients on a per user basis. Something on the lines that Bamboo does.

Are there any out of the box software licensing systems available for SharePoint? or any open source code I can leverage? I am trying to avoid having to write an entire licensing web part / licensing management solution.

Any recommendations are highly appreciated.

Thanks