I would have thought this had been posted here before, but I can't find a reference. A [link|http://blog.sykosopp.com/wp-content/rulethemall.txt|single "Hello World"] file that can be run as sh script, php, perl, tcl, haskell, python, brainfuck or compiled as C/C++.
[1] See the thread on [link|http://thedailywtf.com/forums/thread/79003.aspx|The Daily WTF] if you don't get this.