I'm not sure what software you may want, but you can start by getting ALL the information possible from the user:
HTTP_REFERER (where did they come from?)
SERVER_PORT (what port are the connected to?)
REMOTE_PORT (what port are they coming from on their machine?)
HTTP_USER_AGENT (what browser are they using -- and OS)
Take a look at the php.net docs on Predefined Variables (AKA reserved keys), just incase I missed any here. http://www.php.net/manual/en/reserved.variables.php
Sawmill has a great server stats app (not free) with which you can probably find out what colour a user's kitchen sink is (not really, but it gives you LOTS of specs)