NmapFE for UNIX/Linux is included in the Nmap source. NmapFE for OSX is available at http://faktory.org/m/software/nmap/NmapFE for Windows is under development as part of NmapFE++, a new frontend for Linux, OSX and Windows. Information is available at http://www.insecure.org/nmap/SoC/NmapFE.html