You have one of those laptops which are WiFi enabled and you are itching to get on the net by accessing one of the numerous wireless hotspots dotting your city. But unfortunately, you happen to be a hard core Linux user who catches a cold at the mere mention of the dreaded Windows word. So is there an easy way to put your WiFi card to good use in Linux? Of course there is. Linux has a very good wireless network detector, sniffer, and intrusion detection system in Kismet which supports 802.11 protocol and works in data link layer of the OSI model. Kismet is said to work with any wireless card which supports raw monitoring (rfmon) mode, and can sniff 802.11b, 802.11a, and 802.11g traffic.
Aaron Weiss has put together a simple but very informative article where he lists the difference between active and passive sniffers. He also explains how one can install,configure and run kismet to work with ones WiFi card which makes an interesting read.