Ethercalc
EtherCalc is a web spreadsheet. Your data is saved on the web, and people can edit the same document at the same time. Everybody's changes are instantly reflected on all screens. Work together on inventories, survey forms, list management, brainstorming sessions and more!
Consider using Ethercalc as an alternative to Google Sheets. Registration is not required.
Public instances
The following list is based on what I found while browsing the Internet and the hosts of other free services. Every instance is free -- both gratis and libre.