Libexo is a library for xfce that contains a bunch of additional widgets and a framework for editable toolbars (an improved version of the framework present in gnome), light-weight session management support, functions to automatically synchronize object properties (based on gobject binding properties) and several miscellaneous utility and helper functions for application developers.

while xfce ships with quite a few libraries that are primarily targeted at desktop development, libexo is targeted at application development, with a focus on applications for xfce.