CryptPad
CryptPad is a private-by-design alternative to popular office tools and cloud services. All the content stored on CryptPad is encrypted before being sent, which means nobody can access your data unless you give them the keys (not even us).
Licensing: GNU Affero General Public License (AGPL) v3.0 (Source code)
With its collaborative rich text editor, Markdown-based presentations, and spreadsheet editor, consider using CryptPad as an alternative to Google Docs, Slides, and Sheets, respectively.
CryptPad also has a code editor, kanban system, polls (comparable to Doodle), and whiteboard.
As noted above, all content is encrypted before being sent to the server. Optionally, an additional password can be required to access projects. Registration is not required of any user.
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.