Auto Auto-rotate: per app rotation settings. Free, open source.
Missed notifications reminder: adds a repeating sound or vibration if a watched app has an active notification. Ie, you got an email when you were away from your phone so every few minutes your phone makes a beep or whatever so you don't need to check the screen constantly. Free, open source.
MJ PDF: simple, fast PDF reader. Free, open source.
Snap drop: web based instant file transfer between devices. Free, open source.
X-Plore: the only file manager that ever clicked for me. Dual pane that's really intuitive. Absolutely packed with features (connects with local network, cloud providers, ftp, ssh, dlna, has file sync)