The excelcalcs site has been open as a test site since January 2007 but began inviting users only in March 2007. They are looking for as many users as possible so you could help them out by sending your friends and colleagues there.
You should checkout www.excelcalcs.com. The home page reads "Download our free XLC software which gives MS Excel the capability of displaying cell formulae as mathematical equations. Your worksheets will read like textbooks, they'll be easy to understand and easy to check.