There are a bunch of GNOME/Gtk applications that are useful in an office enviroment.
Evince - Document viewer (official part of GNOME).
Evolution - Groupware and Email application (official part of GNOME).
Gnumeric - Spreadsheet application.
Inkscape - Vector Graphics and Presentation Creations
Ease - Presentations (using Clutter, in development)
GnuCash - financial application
gLabels - label designer
Glom - database designer similar to Filemaker
Dia - diagram creation
Office related Libraries