NETCHESS is free for any use. You can modify source code but you have to add "readme" file and write what you modifacate.
Newest version : 1.2
Windows - binarieshttp://netchess.wz.cz/download/netchess12_win.zip (290kB)
Linux - binaries (i386)http://netchess.wz.cz/download/netchess12_lin.tar (580kB)
If you want compile sources, you will need FLTK www.fltk.org
In archieve is also included DevCpp project file.
* Files with extension .7z is packed in 7-zip format. More informations you can find at www.7-zip.org