There are several free and open-source calendars available for Linux, Windows and Mac OS X. Here are some options you may find useful:
1. GNOME Calendar - a lightweight calendar application that is part of the GNOME desktop environment. It supports various calendar types including lunar/lunisolar and has an option to show 13-months instead of 12.
2. KCalendar - a powerful and flexible calendar application that supports multiple calendars, including lunar/lunisolar and 13-month cycles. It also includes various plugins for additional functionality such as Google Calendar syncing.
3. Calibre - a comprehensive e-book management tool that also includes a calendar module. While it is primarily used to manage books, it can be useful for those who need a calendar with lunar/lunisolar or 13-month cycles.
4. OsmAnd - a mobile app that allows you to create custom maps and offline maps. It supports various map types including moon phases, which may be useful for those who need to plan trips based on the phase of the moon.