The YFP Login Form Customizer plugin adds an input box to the WordPress login screen. The value to be entered is shown above the input field. The idea is that a person will easily figure this out, and a robot will not.

The value to be entered can be changed in the Settings section of the Admin panel. The message can also be changed to some extent.

Simple to use. Installs like any other plugin: just drop it in the plugins folder and activate it.