It looks like Google is getting ready to release a its own online office suite. First it had Gmail, then Google Calendar, and now the 'Google Spreadsheets'. This is a browser based spread sheet that you can perform some basic mathematical operations.
Its no real replacement for Microsoft Excel. Although, if you have a quick list of stuff you have to calculate or a list of things that you want to track, this is a great application for it. Check out this quick tour of the program.