Pocket Informant - Pocket Informant is the Best Calendar app for mac and iOS. Pocket Informant for iOS, Mac desktop, and Android.
Todo.txt - Track your tasks and projects in a plain text file, todo.txt. A todo.
Awesome Cal - Awesome Cal is a Calendar application for your iPhone that you can take anywhere and keep close by to monitor all the upcoming birthdays, festivities, meetings, and more from a single place.
Task Coach - Task Coach is a simple open source todo manager to keep track of personal tasks and todo lists.
Fantastical 2 - Fantastical, the Mac calendar app you'll enjoy using. Quickly create new events with natural language input and more.
Strikethru - Strikethru is a kind of to-do list notebook for managing work lists and staying updated with the important tasks.