Google is working on a spreadsheet product that will be free. Apparently it is pretty primitive now but is available for limited testing.
You can sign up for the test here:
http://www.google.com/googlespreadsheets/try_out.html
Apparently they do want it to eventually rival excel in terms of functionality. One interesting feature it will have is the ability of many people from anywhere to access and work on the same spreadsheet at the same time.