While digging out my inbox, I noticed that Gary Klass had offered several links to Excel charting tricks and add-ins to the POLMETH listserv:
Gary himself has assembled an excellent overview of best-practices when using charts and graphs.
John Peltier, on box and whisker plots, simulated probability charts, dynamic control, histograms, etal. Peltier also offers several tutorials on charting tricks.
John Walkenbach has many add-ins to extend Excel's labeling, chart sizing, and exporting functionalities.
Kelly O'Day offers an excellent overview of Excel's capabilities, placed within a cognitive framework to emphasize the most effective means of visual communication.
Tushar Mehta has compiled an extensive collection of add-ins, tutorials and tips, including Gantt charting, flexible log scaling, and handling arrays.
Andy Pope offers several tricks to get the most out of Excel's graphing functions.