The best Calendar alternative to Roundcube is Kolab, which is both free and Open Source. If that doesn't suit you, our users have ranked more than 25 alternatives to Roundcube and three of them are Calendar Apps so hopefully you can find a suitable replacement. Other interesting Calendar App alternatives to Roundcube are Zimbra Collaboration Suite and OX App Suite.
Kolab provides a FOSS communication & collaboration solution that can be completely self-hosted. Take back control of your data and access it via the webinterface, or existing mobile & desktop clients.
Zimbra Collaboration is an open, secure collaboration tool for email, calendaring, tasks and more. With the most innovative web application available today, Zimbra boosts the productivity of users on any desktop and dramatically reduces TCO compared to legacy platform vendors.
Zimbra Collaboration Suite is the most popular commercial alternative to Roundcube.
The OX App Suite by Open-Xchange is web-based platform offering a suite of apps like an email client, calendar, contacts magement, cloud storage with synchronization and productivity apps for word processing, spreadsheets and presentations.
OX App Suite is the most popular SaaS alternative to Roundcube.
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