Does anyone know a good open-source desktop client (Windows/Linux) that supports sub-tasks?
Thunderbird & Lightning is nice, but the task part is not very nice to use, especially due to the missing sub-tasks.
The Pro version of Rainlendar http://www.rainlendar.net/ works excellent, but is neither Free nor gratis. Still if that isn’t a consideration for you I recommend to have a look (the Pro version can be downloaded and installed, but adds a sort of watermark that it is unregistered).
But actually a more simple desktop widget or simple app of some sort that displays sub-tasks and has CalDav support would be nice to have as FOSS.