I recently used Ruby's
__END__ keyword in a client project.
__END__ denotes the end of regular source code in a Ruby file; nothing below it will be executed.
It's essentially a one-line block-commenting device, which is how we used it when trying to recreate a successful pattern borrowed from somewhere else in the project. Instead of toggling between files, we copied beneath
__END__ the code we wanted to emulate, started building the new method, and cleaned up before committing.
One benefit of this technique is that any code below
__END__ is available via the special filehandle
$ ruby foo.rb