Environment: Ubuntu Desktop Web Browser: Firefox Email Client: Thunderbird Terminal: GTerm IDE: Geany, Eclipse when required File manager: Nautilus Basic Text Editor: GEdit IRC/Messaging Client: Thunderbird (chat) PDF Reader: Evince Office Suite: LibreOffice Calendar: Google Calendar Video Player: VLC Music Player: RhythmBox Photo Viewer: GNOME Image Viewer Screen recording: none; live screen share would be useful, I haven't found any cross-platform that work well yet. In short, most of what you have selected as best-of-breed work really well for me.

Used the way others have already commented; also use as a simple way to publish alert-related things from my own projects to interested clients, both internally and for public consumption. I've tried more "modern" systems for the latter, but nothing is as universal. Simple, well-supported. What's not to like?