Zim Desktop Wiki
Zim is a graphical text editor used to maintain a collection of wiki pages. Each page can contain links to other pages, simple formatting and images. Pages are stored in a folder structure, like in an outliner, and can have attachments. Creating a new page is as easy as linking to a nonexistent page. All data is stored in plain text files with wiki formatting. Various plugins provide additional functionality, like a task list manager, an equation editor, a tray icon, and support for version control.
another Zim fan
Excellent cross platform software, i.e. on Linux, Mac, Windows
By using Markor and Syncthing (or other sync solutions) even in combination with Android devices