Google Spreadsheets was released on limited test a few days ago. Free, of course.
Sign up quick for an invite if you're interested - the last I could tell, Google were sending out invites only a couple of days after signup, so if you sign up now you may be able to get to try it out.
You can import Microsoft Excel .xls files and also .csv files from your computer, and export to those formats too. Plus, you can export or save your spreadsheet as a Webpage, i.e. HTML file, which is handy. I'd like to be able to import Excel files attached to my Gmail direct, without having to save them to my computer first, but no doubt that will come.
Google have rustled up an overview and some help pages about Google Spreadsheets, plus there's also a Google Spreadsheets Google Group.
I've not had the chance to try it out in detail yet but if there's more to report back on, I will. It looks very very good though, and one of the most potentially useful things to come out of Google Labs in a while.