Never Trust Yellow Fruit
You've probably heard about the WiFi Pineapple from Hak5. It's a fascinating device that allows you to do some creepy pen testing. It's the sort of tool that could be used for evil, but it's also incredibly useful for securing networks.
The hardware is fairly basic and resembles an off-the-shelf router. The multiple network interfaces really shine, however, when paired with the operating system. The WiFi Pineapple software creates a rogue, hidden access point that purposefully tricks clients into connecting to it instead of the AP they're usually connected to.
It doesn't stop there, either. The software sniffs the wireless network for clients looking for APs they know, and then creates fake SSIDs that match what clients are looking for. Once the client associates with the SSID it thinks it already knows, the Pineapple provides internet service, but it also sniffs every packet along the way. Scary stuff.
There are many uses for the WiFi Pineapple, and many of them are white-hat sorts of things. Be warned though, because you might be connected to someone else's WiFi Pineapple right now! To get your own piece of dangerous awesomeness, head over to the website now. There is a 2.4GHz-only verison as well as a dual-frequency model.