Hi antideliguy,
I'm not sure what you mean. Do you mean you want a mcahine on your network to intercept web connections coming out from your network and add a pop under / banner? If this is the case, you could look at squid web proxy
http://www.squid-cache.org I'm sure there would be a way to add in a banner while using squid as a transparent proxy.
Hope this helps.
Cheers,
Cleidh Mor