Recommended compilers for najitool and libnaji:

BSD/UNIX/Linux:
GNU Compiler Collection http://gcc.gnu.org/

DOS:
DJGPP, very simular to the above http://www.delorie.com/djgpp/

Windows:
Dev-C++ http://www.bloodshed.net/devcpp.html
MinGW http://www.mingw.org/

UNIX/Windows porting:
Cygwin http://www.cygwin.com/
WINE http://www.winehq.com/

Recommended compilers for naji_gui:

Microsoft Visual C++ 6.0
NOTE: This is not a free compiler, I'm planning to rewrite naji_gui in a free compiler soon.