JS-Ghost contains a MooTools based class and a CSS file for styling. It's primary function is to layer labels over inputs for intuitive, real-estate conscious input labels.
With a brute-simple instantiation, this class doesn't require much tweaking (of which can be done entirely through an additional CSS file to maintain the codebase).
A "ghost" value is retrieved, by default, from the input's title attribute.
Yup! That's it :)
For a real-life example, check out example.html.