Links
First Graph Coloring Challenge (DIMACS): http://mat.gsia.cmu.edu/COLOR/instances.html
Second Graph Coloring Challenge: http://mat.gsia.cmu.edu/COLOR03/
J. Culberson's Graph Coloring Resources Page: http://www.cs.ualberta.ca/~joe/Coloring/
Bibliography on Graph Coloring, by Marco Chiarandini: https://github.com/belzebuu/coloring-verifier
Results on some hard instances, by Daniel Porumbel: http://www.info.univ-angers.fr/pub/porumbel/graphs/
Graph Coloring - Algorithm Engeneering, by Jakub Marecek: http://www.cs.nott.ac.uk/~jxm/colouring/
Exactcolors - Optimal graph coloring via linear programming methods: http://code.google.com/p/exactcolors/
A Graph Coloring Web Application, by Jose Antonio Martin H.: http://graph-coloring.appspot.com/
Certificates
Certificates of some hard instances, by Daniel Porumbel: http://info.univ-angers.fr/pub/porumbel/graphs/tsdivint/
Certificates of other hard instances, by Olawale Titiloye: https://sites.google.com/site/olawaletitiloye/graphcoloring/qacol
Certificates of other hard instances, by Qinghua Wu: http://www.info.univ-angers.fr/pub/hao/ie2col.html