Software that uses openmotif
----------------------------

color_draw    simple drawing program (motif/6A/chapter-11)
cube_motif    rubik's cube sim
fvwm          F Virtual Window Manager
geomview      geometric viewer
ida           image viewer
ImageMagick   image processing
Mosaic        NCSA Mosaic (using CK fork)
mwm           Motif Window Manager
nedit         text editor
xeditor       text editor
xcal          calendar 
xmmix         motif audio mixer
xpdf          pdf viewer

see also 
http://www.motifdeveloper.com/non-comm.html

Software that uses gtk
----------------------

gqview        image viewer
osmo          personal organizer (with calendar)
xchat         irc client 

Software that uses fltk
-----------------------

dillo         web browser