If you want to get around the whole email thing it's very easy, you don't need a serial to get the demo of Stuffit to work, you just need the email to get the dowload url. There are plenty other sites out there that offer Stuffit downloads without having to register, I got mines from somewhere within CNET.

Also Stuffit is a horrible program. It's huge and slow comapred to Rar/Zip which seem much faster and much easier to use.