It’s widely reported that Google will be releasing an online spreadsheet on the 6/6/2006 (presumably US time).
Probably limited signup initially, but keep an eye on spreadsheet.google.com, which is currently showing a 404 (rather than a “Host not found”).
Update 20:15. Sneak peak and sign-up (they’ll email you when it’s available).
Update 22:00. Now at spreadsheet.google.com I get “Server Error. The server encountered a temporary error and could not complete your request. Please try again in 30 seconds.”