Freeware Proxy Server?

Anyone know of a freebie proxy server I can play around with that can handle a mid-size network (50 stations), and might have similar functionality to MS ISA. Using it mainly for Firewall (traffic screening, packet filtering and general port 80 snooping logs). We need to check if any of our users are engaging in …illegal activities so to speak.
Before you write and tell me just to buy ISA server, I’m just looking for a freeware software for now.
Thanks for your help in advance. Mike