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.