Rainlendar
Freemium
Rainlendar is a calendar program for Windows, Mac OS X and Linux.
Kontact - KOrganizer
Open Source
KOrganizer is the calendar and scheduling component of Kontact, the integrated personal information manager from KDE.
Osmo
Open Source
Osmo is a handy personal organizer, which includes calendar, tasks manager, address book and notes modules.
Compare Lightning with alternatives
Compare Rainlendar and Lightning and decide which is most suitable for you.
Compare Kontact - KOrganizer and Lightning and decide which is most suitable for you.