The X toolkit or Xt library provides you with readymade elements, called Widgets, like buttos, labels, drawing areas etc. that can be directly used by your program. They are several popular toolkits available. Some of them are free. The most popular one has been the Motif Widget Library that gives a 3D look to the widgets. Open Motif is a free version of Motif Library.
Example mhello.c is here.