You could use a packet sniffer (ethereal for instance) and parse out all the http traffic, run parser over *that* data and pick out all the web sites..
I don't recommend that though. You'll get a performance hit on your box, and a *heapload* of data.
Much better would be to install Squid as a transparent proxy, have all your machines point to your proxy and log the hits.
You gain the advantage of a nice log file, plus the caching ability of squid to improve the performance of your web browsing users