HTTrack license is available in this file :
\App\WinHTTrack\gpl.txt

Source code is available in this directory :
\App\WinHTTrack\source\source.7z

Source code is available at this address (LiberKey.com) :
http://download.liberkey.com/sources/HTTrack

Source code is available at this address (original address)
http://www.httrack.com/page/2/en/index.html