Not a particularly useful program, but I had an assignment
a while back in a class to write a program in the language
of our choice to count the words in a text file, and print
out the 10 most commonly used words.  It's not very fast,
not very efficient, etc., but was fun to write.  After
beating the program around for a while, I managed to get
it down to:

(Updated: removed unnecessary assignment to %a.  Thanks merlyn)
perl -na0777e 'map {++$a{$_}} @F; print join("\n", (sort { $a{$b} <=> $a{$a} } keys %a)[0..9]), "\n"' <filename>

And after all that, I didn't even get a very good grade.