I am building a website that allows visitors to vote. It is important that visitors do not have to sign up to the site to use it, this opens up the site to abuse. How could I prevent an anonymous user from voting more than once? I can use Javascript to do this but that would be easily worked around. I thought there might be some way to validate the input on the server side but I don't know how to go about this.
Also what are the most common ways in which sites are abused and what can be done about it. I am determined not to make the user sign up to the site and have to log in.