Description: Calendar software. Why should it be included (differentiator): the calendar is just really handy (everything is displayed like a "widget" that is just right there in your desktop), has lots of theming options that make us able to make it look like just like we want and it has almost 50 translations .
Link to source tarball: http://www.rainlendar.info/download/Rainlendar-Lite-2.13.1-i386.tar.bz2
Is not open source.
(ps: Just a suggestion for the pre-installed calendar in solus: it would be better if events were marked with color or in some other way in the raven calendar widget.)