Python, Ruby, Bash, HTML, Javascript tested. Worked.

There is even brainfuck and whitespace in there...

Huh! Somehow I missed whitespace in the source...

Presuming you have the languages installed, this script will download and execute it for each language except javascript and brainf*ck.

  #!/bin/sh

  SRC_FN=poly.c
  LHS_FN=poly.lhs
  BIN_FN=`basename -s .c ${SRC_FN}`
  curl -s -o ${SRC_FN} http://mauke.ath.cx/stuff/poly.html
  cp ${SRC_FN} ${LHS_FN}

  runghc ${LHS_FN}
  bash ${SRC_FN}
  zsh ${SRC_FN}
  ksh ${SRC_FN}
  tclsh ${SRC_FN}
  perl ${SRC_FN}
  python ${SRC_FN}
  ruby ${SRC_FN}
  make -f ${SRC_FN}

  gcc -w -o ${BIN_FN} ${SRC_FN} && ./${BIN_FN}
  gcc -o ${BIN_FN} ${SRC_FN} -trigraphs && ./${BIN_FN}
  g++ -w -o ${BIN_FN} ${SRC_FN} && ./${BIN_FN}
  g++ -o ${BIN_FN} ${SRC_FN} -trigraphs && ./${BIN_FN}

  rm ${SRC_FN} ${LHS_FN} ${BIN_FN}

This is older than the sun. But still clever.

I had never heard of trigraphs before seeing this.

http://en.wikipedia.org/wiki/C_trigraph

I think I'll stay away from them.

Reasoning behind trigraphs (partially explained in footnote in standard, section 5.2.1.1) is that there is something like "Invariant Code Set" which is subset of ASCII and some characters used by C does not fall into this set (which is obvious since C uses all ASCII characters except @ and $, with $ being valid character for identifiers on some compilers). Real usefulness of this feature is exactly zero, but it provides some insight into process of standardization. Uselessness of this is even more pronounced by fact that there is no trigraph sequence for _, which also does not exist in some encodings (althought for slightly different reasons).

Because almost all encodings that are not somehow compatible with ASCII are obsolete now, trigraphs are obsolete too. But it is certainly interesting thing to know about. And by the way "incompatible with ASCII" means that the characters are there, but look different (¤ vs. $, ¥ vs \, and so on...)

...or according to the wikipedia article it was intended to support C code entry on keyboards that don't have characters for [, ], {, }, #, etc. In other words torture.

Actually, this was useful on an the Apple II series where you didn't have {} characters on the keyboard and wanted to program in C.

I used some IBM mainframe keyboards that didn't have [] keys (but the computer did have a C compiler installed).

http://ideology.com.au/polyglot/ is a nice eight-language (Cobol, Pascal, Fortran, C, PostScript, Shell, x86, Perl) polyglot program.

Conveniently, it has a source viewer which will highlight relevant parts of the program's structure for a given language. For instance, the C compiler sees this: http://ideology.com.au/polyglot/c/

Check out this.

http://shinh.skr.jp/dat_dir/fizzbuzz7.gif

It is a valid GIF image file, but also a valid Fizz Buzz program in Ruby, Perl, Befunge, Z80 machine code, and x86 machine code.

Befunge makes it epic, but it could be polycode for the ages if the picture itself compiled to a FizzBuzz program in Piet (http://www.dangermouse.net/esoteric/piet.html)

Now make it valid Malbolge and you win.

Genius!

Can someone please explain why this is clever?

It's a single source file that works in a dozen languages.