EDIT Different names This character is called octophorpe, hash, crosshatch, fence, gate, grid, gridlet, number sign, sharp, crunch, mesh, hex, flash, pig-pen, gate, hak, oof, rake, square, and widget mark.

On phones, the name of the key is »pound key« in America and »hash key« in Britain.

Czech name for # is křížek (little cross).

 jb  :  filip 

EDIT History
There are different theories as to where the character and especially the name octothorpe come from, which seems to be the scientifically correct even if not most publicly known name.

The first and probably correct theory claims that the character was originally used in cartography and that the name stems from it being used as a symbol for »village«, representing a marketplace surrounded by eight fields (Bringhurst 1996, 282)

Another theory is that the character is derived from a shorthand writing of the abbreviation lb which stands for the latin libra (balance). The name supposedly stems from a combination of »octal«, an eight-point pin in electronics, and either the name of an eighteenth century English philantropist, James Edward Oglethorpe, or an Olympic athlete, Jim Thorpe. His name would have been chosen because one of the engineers of Bell Labs who in the 1960s introduced the first touch tone telephones and needed names for the two new keys »#« and »*«, belonged to a group who wanted the return of Thorpe's medals that had been taken from him and given to a swedish athlete (http://dict.leo.org/cgi-bin/dict/urlexp/20040216182730)

 jsw  jsw 

EDIT Typography
The number sign is positioned before the number with a flexible space in between

 jsw