Chip-Firing and Rotor-Routing on Directed Graphs
  We give a rigorous and self-contained survey of the abelian sandpile model and rotor-router model on finite directed graphs, highlighting the connections between them. We present several intriguing open problems. 
Alexander E. Holroyd, Lionel Levine, Karola Meszaros, Yuval Peres,   James Propp, David B. Wilson