I love your application but there is one thing that jumped to my mind which would make it even better (and would cause me to quit using that horrible Lightning-Extension of Thunderbird): an extended Manager.
I think of a new view which looks pretty much like the "iCal"-Application of MacOS (Day, Week and Month perspective + list perspective which just lists the appointments like it is implemented now in the manager).
Of course such a view is not really necessary as most (or all) of the functionality it would provide is already implemented in the normal Rainlendar-Application (the Desktop "widgets") but personally I like a Windows-Style window for serious work (when adding appointments etc.) and the Rainlendar-Widgets mostly for a quick overview or simple tasks.
Considering the fact that most of the logic for rendering appointments in a calendar is already in the application (as there are already month-views, week-views etc.) it should not be too much work to implement such a Window.
It's a pity that there are none outstanding of those "conventional" calendar-applications for Windows. There are a few which are not bad but those lack synchronization (Outlook, Windows Live Calendar) or other key-features (Lightning / Sunbird does have all the features that I need but it feels slow and buggy compared to Rainlendar (or any other native application for that matter)). This last feature - this "conventional" calendar view - would make Rainlendar the one to rule them all
Let me know what you think!
PS: If you decide to implement it, please make the window appear by double-clicking the Rainlendar tray icon (single click for bringing the widgets to the front).