Disclaimer: I am medicated. Using productivity apps is easier when you're medicated.
Anyway, here's some apps that I use:
- Mozilla Thunderbird: Emails, calendars, tasks and contacts in one place. Also the only functional fully offline calendar you can find on Windows. They are in the process of developing an Android version with the developer of K9-Mail. They're also finally getting to a UI update so it doesn't look like shit, which has previously been a dealbreaker for a lot of people.
- Tasks.org: Simple tasks app I use exclusively as a shopping list. It works. That's about it.
- Loop Habit Tracker: Habit tracker I use for my daily/weekly routines. Very simple.