MediaWiki - MediaWiki is a free software wiki package written in PHP, originally for use on Wikipedia.
Followup.cc - FollowUp for email is a lightweight productivity suite that keeps your conversations and tasks from slipping through the cracks. Set perfectly timed reminders that arrive at the top of your inbox, right when you need them.
TiddlyWiki - a non-linear personal web notebook
Gmelius - Gmelius is the first collaboration platform designed for G Suite and your company's daily tools.
Confluence - Confluence is content collaboration software that changes how modern teams work
Boomerang for Outlook - Boomerang for Outlook offers email productivity tools to track, schedule and send mails at the right time.