This week's book giveaway is in the OCMJEA forum. We're giving away four copies of OCM Java EE 6 Enterprise Architect Exam Guide and have Paul Allen & Joseph Bambara on-line! See this thread for details.
You'll need to do this with a cookie (search the forums for many examples). A fairly insecure mechanism, but it will do what you ask (assuming the same user uses the same PC to access your site and they don't clear down cookies).