I had the fantastic opportunity to have a recorded video chat (a vidcast) with Mike Morris the Global Director of Business Development at Endace recently. Endace is in my mind the market leader when it comes to network packet capture solutions. They have really evolved network packet technology from its roots in open source TCPdump and commercially available Wireshark to performing functions and providing value as an always available network packet capture solution.
In Internet security packets are truth. They are the perfect evidence of what was transferred over a network. That said capturing packets requires access to the network medium and storage space.