You should use OpenDNS

If you're not yet using OpenDNS to see what's happening on your network - take it from me - you should.

OpenDNS, the world's largest and fastest-growing DNS service, is free to use and gives you the tools to easily see which Web sites are most visited on your network. If you see problem sites in your network stats, you can easily block them with OpenDNS's industry-leading free Web content filtering.

It also makes your network faster and more reliable. OpenDNS will quickly become your most valued tool in your IT toolbox.

http://www.opendns.com/