http://contextures.com
Contextures - Debra Dalgleish's Excel Tips and Techniques. Deb has lots of information and examples for all of those things you really should know about, but always need to check, including pivot tables, filters, validation, comments, conditional formatting, and more
http://andypope.info Andy Pope's Excel Information, with creative charting solutions and VBA tricks
http://tushar-mehta.com Tushar Mehta's web site, which includes charting content, helpful add-ins, and much more
http://www.jkp-ads.com Jan Karel Pieterse's tutorials and very useful (and free!) utilities, including the Name Manager and AutoSafe
http://www.mcgimpsey.com/index.html
McGimpsey & Associates - John McGimpsey's site has useful information about Excel formulas, VBA, and Security, plus resources for users of Excel for the Mac
http://puremis.net/excel/
Home of Masaru "Colo" Kaji, Colo's Excel Junk Room, and The Road of The Cell Masters
http://www.rondebruin.nl Ron de Bruin has posted tutorials and add-ins for emailing and for entering and filtering data
http://www.nickhodge.co.uk Nick Hodge is developing a variety of tutorials explaining the Excel user interface, charting, and VBA
http://xcell05.free.fr Laurent Longre's site is mostly in French, but his MOREFUNC.XLL worksheet function utility and documentation are available in English
http://www.prodomosua.it/ppage02.html Fernando Cinquegrani has developed a wide array of Excel utilities, mostly involving very creative techniques for displaying data which go well beyond anything Excel's charting capabilities could produce
http://www.dicks-clicks.com Dick Kusleika is better known for the Daily Dose of Excel on Dick's Blog, but there are some gems here on automation of Outlook and working with external data
http://exceluser.com/
ExcelUser - Charley Kyd's categorized portal of Excel links, with emphasis on business applications, and featuring a dashboard approach to display of business data
http://www.thatexcelguy.com/
That Excel Guy - Curt Frye, author of numerous books on Excel, Microsoft Office consultant, and professional comedian.
http://www.edferrero.com/
Ferrero Consulting Pty Ltd - Ed Ferrero offers Excel tutorials, Excel charting tools, and VBA code and utilities.
http://blog.livedoor.jp/andrewe/ Andrew Engwirda's Excel Tips blog, showing Andrew's experiments with many Excel-related features and technologies.
http://www.excelenthusiasts.com/
Excel Enthusiasts - Ashish Mathur answers common questions, discusses best practices, and shares published articles on Excel.
Microsoft Newsgroups These are the extensive newsgroups hosted on Microsoft's own servers, but read and replied to by volunteers, including MVPs like myself. Click on the link provided to access the web interface, or using an NNTP newsreader (such as Outlook Express or Netscape), set up an account using the news server msnews.microsoft.com, then subscribe to any groups of interest. I monitor the microsoft.public.excel.charting group nearly every day, and when time allows I also visit microsoft.public.excel.programming and .misc. I prefer NNTP access, but there are a number of web interfaces, including Microsoft, Google, ExcelTip, to name but a few. Archives of the newsgroups can be searched at Google.
Mr. Excel A well run forum with a clean interface, an extensive archive, and a lively population, this is my second favorite forum, after the Microsoft Newsgroups. The focus is on all things Excel.
VBA Express This forum has an environment similar to Mr. Excel, though it's newer and not nearly as large. It focuses on VBA and related programming, not just in Excel, but in any VBA platform.
It seems blogs are springing up like mushrooms, and some disappear as quickly. Here are some of the Excel blogs that I have been following for a while. I try to remove any that become stagnant.
Daily Dose of Excel - Dick Kusleika's outstanding blog, where Dick and many other Excel experts cover the difficult topics in Excel.
Andrew's Excel Tips - Andrew Engwirda is "an incurable Excel addict" who shares his exercises and experiments in Excel.
Cell Matrix - John Mansfield blogs about his Excel experiences.
Microsoft Excel 12 - A discussion of what's new in Excel 12 with 'Softie David Gainer.
Excel Blog - "This is a blog about Microsoft Excel."
Excel Link Exchanges
http://vertex42.com/ExcelLinks/
The Excel Nexus - The Ultimate Excel Portal Categorized listings of links to Excel resources.