Unique Identifier of Every Drupal User Instance
If you want to uniquely identify browsers, Drupal tracks browser sessions via the sid &amp; ssid column in its sessions table. 

If you're looking to uniquely identify computers, you could try the IP address manager module. 


  IP address manager records user IP addresses, and allows admins to
  view which IP addresses have been tracked against a user, and which
  users have been known to use a certain IP address. This can be
  currently used to identify duplicate accounts.
  
  The aim of this module is to provide a way to identify a user when
  they are not logged in to provide some action or context to their
  presence by maintaining lists of users for some purpose - such as, but
  not limited to, banning them.


The caveats with this approach is, depending on the network setup between the client &amp; host, you might get false duplicate IP addresses for users (e.g. a home network router using NAT).