Manuskript is an open-source tool for writers, with outliner, character management, plot development, distraction-free editor, etc.

Outliner
Organize your thoughts and snippets in a hierarchical manner. Organize them as you wish, reorganize them on the go.

Distraction-free mode
Get rid of all distraction while writing. Set personal goal in word count or time before you can leave editor.

Novel assistant
Use the snowflake method to grow your single genius idea in an coherent whole, with complex characters, intricate plots and detailed universe.