Android: If you often forget to call your parents or make important calls, you need Call Planner. It's a simple app with one purpose: to maintain a list of people you have to call and remind you of that.
Choose a person from your contact list, assign a colour and priority, and decide whether you want a reminder to call them or not. By default, each such entry goes into a to-do list. If you add a reminder, it will go into the "Planned" list.
Sure, you could use any to-do list manager to do this too, but a few features make Call Planner more convenient. Its reminders give you the option to place a call directly from the notifications pane, saving you a few taps -- especially useful with recurring calls. It automatically declutters your to-do list since the people you need to call are handled elsewhere.
Also, you can set Call Planner to automatically add missed calls to your to-do list, and it will also auto-remove finished calls. Finally, the app also auto-adds entries for contacts with upcoming birthdays (as long as those birthdays are noted in your phone book).