When your finished on this site, check out these....
David Wilson's page provides images of the single-source sandpile on various background heights (including negative heights!). Also check out the z→1/z^2 inversion pictures.
Lionel Levine's gallery has nice pictures of models related to the sandpile.
Problems in Graph Theory and Combinatorics : Douglas West's excellent collection of open problems---includes details and status of each question.
Graph Coloring Problems -- The Archive: the online archives of the excellent book Graph Coloring Problems by Tommy R. Jensen and Bjarne Toft.