Your favourite (IT-related) tool?
The rules, before we begin:
1) You only get to choose one
2) It can be anything you could conceivably use in the practise of IT, be it a software tool or a hardware one.
3) It should be specific (Leatherman Skeletool is good, "Open Source Software" is too broad.).
4) While there may be niche uses for hippos, black cockerels and candlesticks while performing the black arts at stupid o'clock, pick something else :)
5) You don't talk about Robot Club
My choice is OpenSSH for fixing remote kit wherever I am.