Motif is the long-time quasi-standard GUI toolkit (as defined by the IEEE 1295 specification) used on Unix systems.
It is not all that well-regarded, and is very creaky. New Unix and Linux software tends to be written using toolkits actually available under "free" licenses like the LGPL; popular are Qt (using C++?) and GTK (using C, but having a vast army of bindings including C++).
Source licenses used to be really expensive; source is now available under an "open" license that is not open source.
Notable applications that have used Motif include Emacs and Netscape Navigator.
Motif is available for free these days from open motif. Lesstif provides an LGPL re-implementation. The O'Reilly Motif books are also available for download here.
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