Multi-user spreadsheet server.
The EtherCalc online service encrypts your data transmission with HTTPS, but data may be read by anyone who can guess the URL. Download and run your own EtherCalc server if you care about privacy.
Disclaimer: Using the recommended projects on this site will not guarantee that 100% of your communications will be shielded against surveillance states. Please do your own research before trusting these projects with sensitive information.