Apps with 'Plugin system' feature

Joplin is a free, open source note taking and to-do application, which can handle a large number of notes organised into notebooks. The notes are searchable, can be copied, tagged and modified either from the applications directly or from your own text editor.

Micro is a terminal-based text editor that aims to be easy to use and intuitive, while also taking advantage of the full capabilities of modern terminals.

Duality is a modular 2D game engine / editor combo, written entirely in C# and built on top of an OpenGL backend.