http://www.eusprig.org/ - European Spreadsheet Risk Interest Group, a group of academic and business professionals dedicated to highlighting some of the dangers lurking in commercial spreadsheets.
http://www.sysmod.com/ - Patrick O'Beirnes website, home of scanxls and plenty of software development and spreadsheet errors (hes the author of Spreadsheet Check and Control)
http://www.louisepryor.com/show.do?page=home - Louise Pryors spreadsheet quality site.
http://www.exceluser.com/index.htm - Charley Kyds Excel site, lots of useful information on integrating Excel with OLAP.
http://blogs.msdn.com/excel/ - All the latest Excel 2007 info from David Gainer, a program manager on the Excel team.
http://www.j-walk.com/ - John Walkenbachs site, loads of great Excel content and plenty of links.
http://www.excelkb.com/ - xlDennis website content focused towards the more technical aspect of Excel like database manipulation.
http://www.appspro.com/ - Rob Boveys (joint author of PED) consulting site, home of his very useful Excel utilities.
http://www.oaltd.co.uk/ - Stephen Bullens (joint author of PED) consulting site, plenty of good excel content including lots of free tools and utilities.
http://www.decisionmodels.com/index.htm - Charles Williams site discussing the full depth of Excel calculation issues, incredible amount of information and home of FastExcel.
http://www.andypope.info - Andy Popes web site, lots of great Excel and data presentation content.