KDE - https://community.kde.org/Get_Involved - Because KDE Plasma is the desktop environment I love and use the most and also I like a lot of KDE applications very much. Also the KDE community is one of the most friendly and helpful ones I encountered in the FOSS world and are working on many exciting projects/software.
This is a hard question. Before coming there I wasn't even aware of such tools existence, but if we consider me as someone working with such tools (i.e posthog/or) I would probably try to contribute.
Right now I'm planning to do some contribs to Lemmy because I'm really liking the concept