Greenlee GT-11
I have used several types of these non-contact voltage detectors. They are an essential safety tool for anyone who works around live wires, or wires that may be live, not just electricians.

Along with similar products in this price range, what I like about this product is: it's compact size which allows it to be easily carried and accessible; the inclusion of a light indicator (useful in dark places like attics and basements) AND an audible indicator (useful outdoors and in bright lights). Note that some competitive products only glow when current is present.

Features that I like specifically about the Greenlee GT-11 include: the positive On-Off switch that eliminates the need to constantly squeeze a contact point on some other models; and the use of common AAA batteries. Another brand of detector I owned used small watch/button batteries that are harder to find and cost almost as much as the purchase price of the whole unit to replace.

The GT-11 will identify individual hot wires, but may not detect current when the hot wire is buried in a bundle of wires or cables, or when it is encased in conduit or other shielding. As noted in another review, it will only indicate when inserted in the 'hot' opening of an outlet, not in the neutral opening (no current). Some users will find this to be a helpful diagnostic advantage. Others may want a detector that lights up anywhere near a live wire, without needing to identify individual conductors. This is a sensitivity issue.

Greenlee also makes a Model GT-16 with adjustable sensitivity down to 5 volts if that is better suited to the user's needs.

How can they sell these things and get away with it?
I bought this Greenlee GT-11 voltage detector and was very dissappointed. I probed a live outlet and got no reaction until I inserted the tip directly into the outlet ( it only sounded the alarm on one side of the outlet and not the other ). I then tapped it on my hand and the alarm sounded?!?! I have had other voltage detectors that react when placed near the electrical source...just like you would expect. The Greenlee GT-11 is just plain dangerous ...you really never know if the line is live or not.