Kevin Kelly's Cool Tools highlights a tool, a non-contact voltage sensor, that I've found to be invaluable each time I try to figure out the maze of wiring that's tacked up to our basement joists. The one that the site features is the Fluke VoltAlert; when I was stocking up on new-homeowner tools, I went with the GB GVD-504A Circuit Alert, at less than half the price (and eligible for Amazon Prime or super-saver shipping). If you find yourself needing to work with electricity, it's a no-brainer that you need one of these in your toolbox.