MooWatermark is a project mainly written in ..., it's free.
mootools based watermark
Help your users by providing a disappearing text right in the text field.
MooWatermark is essentially very easy to use
HTML
<input type="text" name="watermarkname" id="watermarkname" />
JavaScript
new MooWatermark({
textField: 'watermarkname',
hint: 'Your name please'
});
If you wish to customise the style of the watermark, just add your own CSS class. A custom CSS class can be applied to the watermark by means of passing an additional watermarkClass attribute.
new MooWatermark({
textField: 'watermarkemail',
hint: 'Your email please',
watermarkClass: 'watermarkGreen'
});