Open source note taking and to-do app with synchronization
...Supported cloud services include Nextcloud, OneDrive, Dropbox and WebDAV.
Joplin is available for Windows, Linux, macOS, iOS and Android, with three types: desktop, mobile and terminal. All of them have similar user interfaces and can be synchronized with each other.
Cosas is a web emulation of the GTD app Things. My plan is to use the web interface at work where I can't use my phone. There will also be an implementation for the iPhone, Android and perhaps a thick client if the mood strikes me.
The web interface is written using Ext/JS 4 and all information will be updated using tiny AJAX calls. This is because I want the back end to be as simple as possible to re-implement using whatever framework that is most convenient.