It converts NBO's plot files (*.31 ~ *.41) into Molden format, which can be plotted using Molden, GabEdit, MacMolPlt, JMol, and so on. Only s, p, d, f, and g functions (either spherical or Cartesian) are supported by Molden.
Examples of applications can be found in W. Zou, D. Nori-Shargh, and J. E. Boggs, On the Covalent Character of Rare Gas Bonding Interactions: A New Kind of Weak Interaction, J. Phys. Chem. A, 2013, 117, 207-212 and the erratum J. Phys. Chem. A, 2016, 120, 2057.
Note that starting from version 6 (> 2014), NBO can generate Molden files by setting up $NBO MOLDEN=... $END.
Version 1.0.3 (12/07/2015). Bug fix for d-, f-, and g-functions.
Version 1.0.4 (12/16/2015). Plot file 41 generated by NBO6.
NBO3: http://www.ccl.net/cca/software/SOURCES/FORTRAN/nbo/index.shtml
NBO6: http://nbo6.chem.wisc.edu
Molden: http://www.cmbi.ru.nl/molden/molden.html
GabEdit: http://gabedit.sourceforge.net/
MacMolPlt: http://brettbode.github.io/wxmacmolplt/