Joplin - Joplin is a free, open source note taking and to-do application, which can handle a large number of notes organised into notebooks. The notes are searchable, tagged and modified either from the applications directly or from your own text editor.
Outlook4Gmail - Outlook4Gmail provides easy synchronization of your Outlook and Google accounts
OneNote - Get the OneNote app for free on your tablet, phone, and computer, so you can capture your ideas and to-do lists in one place wherever you are. Or try OneNote with Office for free.
Boomerang for Outlook - Boomerang for Outlook offers email productivity tools to track, schedule and send mails at the right time.
Standard Notes - A safe place for your notes, thoughts, and life's work
BlueMail - BlueMail is an email management app for Android, iOS, and Windows.