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.

Task Coach - Task Coach is a simple open source todo manager to keep track of personal tasks and todo lists.

BlueMail - BlueMail is an email management app for Android, iOS, and Windows.

Strikethru - Strikethru is a kind of to-do list notebook for managing work lists and staying updated with the important tasks.