This started as a fun project between me and my partner - where we wanted to rank the mince pies we ate over christmas. What was interesting was the second we mentioned the idea to other people, they were interested too.
We use a simple authentication cookie once a user is logged in, and only store a user identifier (usually an email) so that users know what pies they have ranked. We store no other user data than this, and I've got no intention of storing more.
Source code is publicly available on Github.