GoodNotes - GoodNotes lets you take notes and annotate PDF documents.
Plot Factory - Online story planner, organizer, and writing application.
Epsilon Notes - Common Mark editor for Android and Windows
World Anvil - World Anvil is a worldbuilding tool for Authors, Storytellers and worldbuilding lovers.
Notebooks by DroidVeda LLP - Notebooks is an app by DroidVeda LLP that comes up with features to help you in creating customizable digital notebooks so you can take notes as well as can also attach drawings to notes.
Plottr - The #1 rated visual book planning and series bible software for professional and aspiring writers.