See http://www.xcrysden.org/doc/XSF.html for an in-depth description for the file format. Example file is attached.
It is used at least by xcrysden, which is somewhat widely used in electronic-structure calculation of solids - and some of the programs such as Fleur (www.flapw.de) also output the structure/density/potential in this format.
xsf file for a SLAB with atoms and potential