Todoist - Todoist is a to-do list that helps you get organized, at work and in life.
Outlook4Gmail - Outlook4Gmail provides easy synchronization of your Outlook and Google accounts
Todo.txt - Track your tasks and projects in a plain text file, todo.txt. A todo.
Boomerang for Outlook - Boomerang for Outlook offers email productivity tools to track, schedule and send mails at the right time.
EssentialPIM - EssentialPIM is a free Personal Information Manager that keeps up with the times and lets you manage appointments, tasks, notes, contacts, password entries and email messages across multiple devices and cloud applications.
BlueMail - BlueMail is an email management app for Android, iOS, and Windows.