NebuAd is a highly controversial behavioral targeted advertising system implemented by many leading ISP's, the sophisticated deep-packet inspection appliance based system at your ISP level analyzes customer's websurfing habits delivering micro-targeted advertising to them, the overall approach used to intercept, analyze and alter the visited webpage's html makes it no less then a Spyware and is raising substantial privacy questions.
"Upon reviewing the record of TCP packets from Google’s server, it is observable that that an extra packet appears in the data stream before the data stream closes. The contents of
this added packet are added to the code underlying Google’s Web page. The added packet
below) indicates that this packet is a forgery and did not come from Google, but from
some other point within the network."