I imagine simply assigning your users a unique key as a cookie would get you 90% of the way there. The chances are good that if a person is returning to your w

I imagine simply assigning your users a unique key as a cookie would get you 90% of the way there. The chances are good that if a person is returning to your website then they are using the same browser. The 10% you miss are cleared cookies and different browsers/private browsing. This could be seen as a feature however since extra anonymity is gained through existing means.