LessTif is the Hungry Programmers' version of OSF/Motif®. It aims to be source compatible meaning that the same source code should compile with both and work exactly the same! LessTif is "free software": it is licensed under the GNU Library General Public License (LGPL). You might also want to check out The Open Source Web for a little more information about the Open Source philosophy in general.
The current version of LessTif is 0.95.0 as of June 10, 2006. The code is available for download in various packages.