What is this very short power cable called?

Solution 1:

I would just call it a "very short extension cord", and in fact a Google search for "short extension cord" turns up lots of results of exactly what you're looking for. E.g., these, which have a pass-through plug.

Solution 2:

"pigtail" is a common term for these (actually any 6" to 1' power cord) in the datacenter environments I've worked in.

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Solution 3:

You can also get short Y power cords.

These cables (including the single variety) are sometimes called "outlet savers".

I also love the 90° rotating plug variety for some applications. The cord goes flat against a wall.

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Solution 4:

I think the official name is "Power Strip Liberator".

Google image check: http://www.google.com/images?q=power+strip+liberator

UPDATE:

Sorry, I think that's a product's name (trademark), not the common name for the "thingy." But I looks like that the product is the "Coke" amongst power extension cords. :)

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Solution 5:

It is called a pigtail extension cord. http://www.mockett.com/pt1-90.html