To-do apps range from simple lists to full task managers with projects, tags, due dates and review workflows. The key differences are how they sync, whether there are iPhone and iPad companions, natural language date entry, and integration with Apple Reminders and Calendar.
Things is a polished native app sold as a one-time purchase per platform, and OmniFocus is a deep task manager built around GTD. Microsoft To Do is free and syncs with Outlook tasks, TaskPaper keeps tasks in plain text, and Super Productivity is an open source option with time tracking. Many apps in this list are on the Mac App Store.