Fourth of July Edition: Print lines of a file containing multiples of a specific number
Perl, 67 chars

open F,"input.txt";print grep/(\d+)(?(?{!$^N+$^N%$ARGV[0]})(*F))/,&lt;F&gt;


Please note that the given character count is taking advantage of one of Perl's more horrific features, namely that a special variable of the form $^X can be written in two characters instead of three, by replacing the ^X with a literal ctrl-X character.

And, of course, this solution wouldn't be possible without a couple of Perl's arguably-equally-terrifying regex extensions that allows one to embed actual code inside of a regex pattern. (But at least those features are clearly documented as being potentially scary.)

[EDITED to fix bug in argument-handling due to not reading the description carefully enough.]