Useful links:

• The Spreadsheet Page: This page is maintained by my favorite spreadsheet guy - John Walkenbach. I have learned quite a bit from his power programming book.
  
  
• Mr. Excel: Great message board
  
  
• Ozgrid: These people sell bunch of useful excel add-ins
  
  
• Excel Help Portal: I haven't used it...but it sounds cool
  
  
• ZoneO Software repository: Some pretty useful stuff for web sites. I usse their freeForum and phpTrafficA
  
  
• PHPJunkYard: Some pretty cool stuff for web sites.
  
  
• Excel plotting techniques: This page explains how VTools creates box plot, normal probability plot and S curve. Useful if you want to create these graphs without using VTools.