Here’s a nice little tutorial from Dexter Industries. In it, they use a Raspberry Pi, their GrovePi add-on board, an LCD screen and a buzzer to create a system that will alert you when an open wifi network is found. Please note: this pushes the bounds of legality a little bit, but (as the tutorial says) as long as you find the network and don’t use it without permission, you’re fine. Read it and get the code here.