Kismet - An excellent tool for your arsenal.

Kismet is an 802.11 layer2 wireless network detector, sniffer, and intrusion detection system. Kismet will work with any wireless card which supports raw monitoring (rfmon) mode, and (with appropriate hardware) can sniff 802.11b, 802.11a, 802.11g, and 802.11n traffic. Kismet also supports plugins which allow sniffing other media such as DECT.

Kismet identifies networks by passively collecting packets and detecting standard named networks, detecting (and given time, decloaking) hidden networks, and infering the presence of nonbeaconing networks via data traffic.

Driftnet - See what pictures people on your network are looking at.

Inspired by EtherPEG (though, not owning an Apple Macintosh, I've never actually seen it in operation), Driftnet is a program which listens to network traffic and picks out images from TCP streams it observes. Fun to run on a host which sees lots of web traffic.

In an experimental enhancement, driftnet now picks out MPEG audio streams from network traffic and tries to play them. You can also now use driftnet with Jamie Zawinski's webcollage, so that it can run as a screen saver.