Standard format for representing large graphs I am aware of the "famous" DIMACS graph format (which frankly looks a little clunky to me - "c" for a comment line ?) and the METIS file format. 

While it's not particularly hard to invent my own graph format, it's nice to follow something accepted so that reuse of code for generating examples and testing algorithms is easier. 


  Is there a standard accepted format for representing graphs for use
  with testing graph algorithms ?
I do not think there's a single standard format. igraph library (http://igraph.sourceforge.net/) could be of use, however, as it supports reading/writing of number of different graph formats.