It tends to be used in old software ( _i.e._ software which has been around for a long time, not necessarily obsolete software); for example:
* Alliance
* the Common Desktop Environment
* DDD (OK, that one's probably obsolete)
* Geomview
* Grace
* of course, the Motif Window Manager
* NEdit
* OpenDX
* Xpdf before version 4
For some reason lots of "scientific" software still uses Motif: ARB, ESO-MIDAS... And some software still supports Motif, although by default it doesn't use it any more: examples include Emacs and Vim.
The MotifZone wiki has a list of Motif applications, but many of the links there are broken.