<p>What is the easiest cross platform widget toolkit? I'm looking for one that minimally covers Windows, OSX, and Linux with a C or C++ interface.</p>

Are we talking GUI Widgets?  If so, I can suggest 3
FLTK:
http://www.fltk.org/
GTK:
http://www.gtk.org/
QT:
http://trolltech.com/products/qt/