Joplin is a free and open source note taking and to-do application available for Linux, Windows, macOS, Android and iOS. Its key features include end-to-end encryption, Markdown support, and synchronization via third-party services like NextCloud, Dropbox, OneDrive or WebDAV.
Turtl can be used to take notes, save bookmarks, store documents and images, and anything else you may need, in a safe place. There are Turtl applications available for Linux, Windows, macOS and Android, while an iOS application should also be available in the future. Chrome and Firefox extensions are available to easily bookmark the page you're on, great for quickly saving sites for later.
The Turtl developers offer the service (hosted server) for free, but a premium service is planned for the future. However, the Turtl server is free and open source software, so you can install and use your own instance.
NixNote2 is a note-taking app that can sync with Evernote (which is available for Android). Nixnote2 2.1 was recently forked from Nixnote and includes new features and many bug fixes.