Berkeley Logo is a freeware interpreter written by Brian Harvey along
with several students (Major contributors are Daniel van Blerkom,
Khang Dao, Michael Katz, Douglas Orleans, and Sanford Owings.) It's
available for Unix/Linux, MacOS X, Windows - complete with C source
code. A version for One Laptop Per Child is also available (and also
older version - BLogo - is available for MS/PC-DOS). If you think Logo
is just a graphics language for little kids, check out sample programs
that should impress you.
