Input text ... dissappear

Tested with MooTools 1.2

Create a input field similar to the following

<input type="text" name="search" id="search" value="Search" />

Then your Javascript

window.addEvent('domready', function() {
    $('search').addEvents({
        'focus': function() { if(this.value == 'Search') this.value = ''; },
        'blur': function() { if(this.value == '') this.value='Search'; }
   });
});

If you want something a little more fancy and dynamic, I would recommend OverText

http://mootools.net/docs/more/Forms/OverText