DC-10Tech, an APU is an Auxilliary Power Unit. Anything is possible
I believe British Aerospace uses four of these under the wings of the BAe146 range of commercial jetliners, although some would say they are actually lawnmower engines as used on the B-17 (re: L-188 above).
Seriously, quite an important piece of equipment. Here in Australia for instance, there are some airports in the "Outback" that do not have ground power, and have very few flights per day (some only one flight per day). As these remote areas are often very hot, airlines could not afford to send an aircraft with a non-functioning APU (air conditioning requirements on the ground). This can make scheduling quite interesting.
Some of the stats of the APU are phenomenal. I am sure someone can correct me with exact figures here, but some can generate enough electricity to run a small town, and spin at about 40,000 RPM.