A hex-shaped (i.e., six-sided) wrench, designed to fit the sockets of certain screws, set-screws and bolts. In the shop, serious woodworkers use them virtually every day. They come in Metric and English sizes.
This site has more info
According to English professor and amateur woodworker, Dr. Philip Leon, "the Allen screw and wrench (also called the Allen key, hex key or hex head wrench), was trademarked in 1943 in the United States by the Allen Manufacturing Co. in Hartford, Conn" It evidently existed earlier in Europe by about three decades. Dr. Leon adds that "[a]lthough H. M. Allen, owner of the Allen Manufacturing Co., did not invent this screw and wrench, his name became the eponym for the adapted American version".
The earliest use in Google News Search is 1952, where several newspapers contain ads for 98 cents sets "allen screws and wrenches." The image on the left reprints a portion of a full page ad -- Kingston (NY) Freeman, 11-03-1948, page 14 -- for a large hardware store.
The Shopsmith Mark package includes sturdy, steel legs that comprise a bench for the machine. Accessories include a 10-in. combination saw blade, a 12-in. sanding disc, a miter gauge and rip fence, an extension table, a tailstock, tool rest and lathe centers, 1/2 inch chuck and key, and an Allen Wrench.
Sources: R.J. DeCristoforo “Return of the Shopsmith”
Illustrated 71 October 1975, pages 96+;
top image, Robert Scharff, Easy Ways to Expert Woodworking New York: McGraw-Hill, 1956;
R.J. DeCristoforo “Return of the Shopsmith", Mechanix Illustrated 71 October 1975;
Philip Leon, "Name Brand Tools", Popular Woodworking, December 2006;p. 104