ajaxXLS a web-based Excel reader has been launched by Michael Robertson. Check it out. From the website:
On this first release of the ajaxXLS viewer, you can expect to open spreadsheet files with all your common cell formatting categories such as text style, decimals, currencies, date, and time. ajaxXLS will preserve your formulas, backgrounds and borders from the files created in Microsoft Excel. ajaxXLS is platform independent and can be used with any operating system.
Note, its platform independent but not broswer independent :p