JSValidation Kicks Butt - Validation Library for Prototype.js

to get it to work, simply include the library like so:

<script language="javascript" type="text/javascript" src="js/jsvalidate/jsvalidate.js"></script>

Add some css styles:

.jsvalidation { color: #ff0000; font-size: 14px; }

add "jsrequired" (for a required field) and the jsvalidate type (such as "jsvalidate_number") to the "class" attribute of your text element.

<input type="text" name="year" id="year" size="4" class="jsrequired jsvalidate_number" />

and finally set the message to be displayed when the validation fails in the "alt" attribute of the element:

<input type="text" name="year" id="year" size="4" class="jsrequired jsvalidate_number" alt="Please enter a Year" />

More on the jsvalidation types at the docs