Detecting surrounding electronics or electrical devices
One cheap idea is to use an IM-me as a sub-1Ghz spectrum analyser.

The Wi-Spy is a cheap 2.4GHz spectrum analyser, which can help characterise WiFi and ZigBee.

A fun site to look at is http://1010.co.uk/org/scrying.html

They have a range of projects designed to pick up signals.