Licensing systems

Hi everyone,

My apologies for posting this here, but I can't think of another place to get independent advice.

I'm looking for a licensing system for my software.
Something that's easy to use for a user and preferably not too easy to hack.
A key factor here would be that licenses would be checked by a third party server (so not run by me but by the licensing company).
I'd prefer not to have to dish out licenses myself over e-mail.

Can anyone give me any tips here?
I can obviously use google, but given the abundance of licensing systems out there with no way for me to tell the long-term implications of chosing a particular system, I felt it best to ask people with some experience in this matter.

Thank you in advance.

PS:
I'm really sorry for asking this in a C++ forum, but it's just really hard to find a group of people who might have knowledge of this subject otherwise.