Bricklink Color Reference Chart
A pretty complete color chart comes from the largest LEGO marketplace - Bricklink. It lists all of the colors they have inventoried along with relevant statistics. A handy guide when you want to plan your build. As of this writing, there are more than 170 piece colors ever produced. If you want to a more complete reference, complete with cross reference between different sources, go to Ryan Howerter's color chart.
