EmSniff is an application that helps you to capture network packets. It differs from TCPDump in that it runs as a service instead of an application. This makes it possible to capture packets in the background. This can help applications like Wireshark to be more secure since they don't have to run with root privileges anymore. They only have to connect to the EmSniff server via libemsniff.
KSnuffle is a network packet sniffer for KDE. It supports multiple concurrent sniffers, packet filtering (aka tcpdump), event triggers and commands, text and binary logging (and replay), remote sniffing (via a demon process), and dynamically loadable plugins. Network traffic is displayed graphically and in detail; supplied plugins summarise traffic by direction (incoming, outgoing), source, and destination.
ng_fwdswitch is a FreeBSD kernel module based on the netgraph infrastructure that works as a basic packet forwarding engine. It will classify inbound IP packets using their source or destination address and forward them to the appropriate outbound interface. It could be used as an IDS helper, say to split "coherent" traffic across several sensors.