The following apps are ones I’ve either used or have heard about from other writers. Each app offers help with productivity, promotion, creativity, or the actual act of writing a story. They are listed in no particular order.
I’m linking each of these apps to the Apple store, but many are also available at the Google Play and/or Amazon App Stores.

1) Interval Timer: Focus on your productivity and beat procrastination. By Nasa Trained Monkeys

2) Evernote: Capture notes and sync across all devices. Stay organized. By Evernote

3) Microsoft OneNote: Lists, photos, and notes, organized in a notebook. By Microsoft Corporation

4) Microsoft Word: A word processor. By Microsoft Corporation

5) Pages: A word processor. By Apple

6) TheBrain: Dynamic Mind Mapping, Note Taking and Information Management. By TheBrain

7) SimpleMind+ Intuitive Mind Mapping: Helps you organize your thoughts, remember things and generate new ideas. By xpt Software & Consulting B.V.

8) Lists for Writers: Ideas for creative writing. By Thinkamingo

9) Thesaurus Rex: a thesaurus with modern content and synonyms for new words. By Dictionary.com

10) Merriam-Webster Dictionary By Merriam-Webster, Inc.

11) Wunderlist: To-Do List & Tasks By 6 Wunderkinder

12) Hootsuite: Schedule Posts for Twitter, Instagram & More. By Hootsuite Media Inc.

13) Pocket: Save Articles and Videos to View Later. By Read It Later, Inc

14) White Noise: Ambient sounds of the environment to help you stay relaxed and focused while you write. By TMSOFT

15) Yoga Studio: Play, create, customize and schedule easy-to-follow HD video yoga classes. By Gaiam, Inc.