Here's a short extension cord version of one

The explanation of why they're a bad idea is valid though. Usually the kinds of people who know just enough to want this type of plug are also the type of people I wouldn't trust with one.

Edit: I'll go ahead and explain why people want plugs like this. These can be used to backfeed power from a generator into a house, letting you used different outlets/lights in the house. A knowledgeable person can actually use something like this safely, but most people who would use something like this don't know enough to do it correctly. The dangers about doing this are:

• You can easily shock yourself off of the exposed pins of the cord. You can avoid getting shocked by waiting to plug into the generator until the other end is connected first.
• You can backfeed power onto the electrical grid. The power going backwards through the transformers will step up the voltage to thousands of volts, and could seriously injure or kill the linemen who are trying to fix the outage. This is avoided by making sure you've opened either the house's main disconnect breaker or the breaker for the individual circuit you're plugging into.

There are some safer ways to setup this type of generator use, there are special generator power cords/outlets that won't have exposed energized prongs if plugged into a generator. There are also transfer switches or breaker interlock kits designed to prevent someone from being able to backfeed power by requiring them to disconnect commercial power before the generator can be fed into the house's distribution panel.