Very useful Excel related websites:

 

Contextures (clear and concise with tons of useful info and examples)

 

McGimpsey & Associates (very useful Excel section)

 

Pearson Software Consulting Services (useful info about functions and VBA)

 

David McRitchie (lots about everything)

 

Peltier Technical Services (if you ever needed to know anything about charts)

The New Guidelines for Writing Spreadsheets (about spreadsheet design delivered in a very candid language)

xlDynamic.com (some useful utilities and a thorough explanation of the inner workings of sumproduct)

Ron De Bruin's Excel page (some great utilities and a very useful code section)