Perl and Ruby have this. In Perl it's kind of an abuse of the perldoc system, like this:

    print "foo"
    =pod
    print "won't run"
    =cut

Ruby basically copied it, although I don't think these have documentation uses:

    puts "foo"
    =begin
    puts "won't run"
    =end

When I get to choose, I prefer sticking to // in C code so that I can use /* ... */ for hiding large blocks of code . . . although #if false ... #endif work too.