The World Wide Web has dozens of excellent sites devoted to Excel. I list a few of my favorites here.
http://www.j-walk.com/ss
This is my own Web site, which contains files to download, developer tips, instructions for accessing Excel Easter eggs, spreadsheet jokes, an extensive list of links to other Excel sites, and information about my books.
http://www.DailyDoseOfExcel.com
This is a frequently updated Web log created by Dick Kusleika, with about a dozen contributors (including me). It covers a variety of topics, and readers can leave comments.
http://www.peltiertech.com/Excel
Those who frequent the microsoft.public.excel.charting newsgroup are familiar with Jon Peltier. Jon has an uncanny ability to solve practically any chart- related problem. His Web site contains many Excel tips and an extensive collection of charting examples.
http://www.cpearson.com/excel.htm
This site, maintained by Chip Pearson, contains dozens of useful examples of VBA and clever formula techniques.
http://www.bmsltd.co.uk/excel
Stephen's Web site contains some fascinating examples of Excel code, including a section titled "They Said It Couldn't Be Done."
http://www.mvps.org/dmcritchie/excel/excel.htm
David's site is jam-packed with useful Excel information and is updated frequently.
http://www.MrExcel.com
Mr. Excel, also known as Bill Jelen, maintains an extensive site devoted to Excel. The site also features a message board.