Horde - Horde Groupware is a free, enterprise ready, browser based collaboration suite.
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.
Roundcube - Web-based IMAP email client
FollowUpThen - The simplest way to schedule an email reminder
ProtonMail - Secure email with absolutely no compromises. Get your free encrypted email account today.
Gmelius - Gmelius is the first collaboration platform designed for G Suite and your company's daily tools.