Web Browser: Firefox, Opera
Email Client: Geary, Thunderbird, pantheon-mail (when the new version is ready)
Terminal: gnome-terminal, pantheon-terminal
IDE: ???
File manager: Nautilus, Thunar, pantheon-files
Basic Text Editor: gedit, scratch-text-editor (from elementaryOS)
IRC/Messaging Client: Telegram
PDF Reader: envince
Office Suite: Libre
Calendar: gnome-calendar (please please please with Caldav support for posteo)
Video Player: totem, mpv (gnome-mpv)
Music Player: audacious
Photo Viewer: gnome-viewer
Screen recording: ???