While Xfce ships with quite a few libraries that are primarily targeted at desktop development, exo is targeted at application development, with a focus on applications for Xfce.
exo 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.
All packages Packages in Distributions
exo source package in Vivid
Version 0.10.4-1 uploaded on 2015-03-22
exo source package in Utopic
Version 0.10.2-4 uploaded on 2014-09-07
exo source package in Trusty
Version 0.10.2-3ubuntu1.14.04.2 uploaded on 2015-03-05
exo source package in Saucy
Version 0.10.2-2 uploaded on 2013-05-22
exo source package in Raring
Version 0.10.2-1 uploaded on 2012-12-30