Outlook4Gmail - Outlook4Gmail provides easy synchronization of your Outlook and Google accounts
Freeplane - Freeplane is a powerful and free software for building mind maps.
Boomerang for Outlook - Boomerang for Outlook offers email productivity tools to track, schedule and send mails at the right time.
vym - VYM (View Your Mind) is a tool to generate and manipulate maps which show your thoughts.
BlueMail - BlueMail is an email management app for Android, iOS, and Windows.
Linked Ideas - A macOS/iOS apps to treat ideas as links of concepts.