Orage!

Rainlendar is a feature rich calendar application that is easy to use and doesn't take much space on your desktop. The application is platform independent so you can run it on all major operating systems: Windows, Linux and Mac OS X. The appearance can be customized with skins and you can even mix different skins together. It is possible to use Rainlendar with other calendar applications too. Rainlendar uses the standard iCalendar format to store its events and tasks so you can easily transfer them between applications. You can also subscribe to online calendars including the Google Calendar and edit your Outlook appointments directly in Rainlendar*. »

gsimplecal is a lightweight calendar application written in C++ using GTK2. It was intentionally made for use with tint2 panel in the openbox environment to be launched upon clock click, but of course it will work without it.