A very simple validation plugin for jQuery in just over 100 lines.

jquery.validator.js

(function(jQuery) {

    jQuery.fn.validator = function(options) {

        var isEmpty = function(obj) { return obj == null || obj.length == 0; };

        var opts = jQuery.extend(true, { // Allows for overriding any messages or validator functions
            wrapperClass: "invalid", messageClass: "message", useMessages: true,
            onValidation: null, onValidationFailed: null, // Callbacks which recieve an object containing the element and state { el: el, state: state }
            validateOn: "blur", // The jQuery events that cause validation to occur
            defaultMessages: {
                "required": "Required",
                "min": "Value is not long enough.",
                "max": "Value is too long.",
                "accepted": "Must be checked.",
                "number": "Must be a number.",
                "date": "Must be a valid date.",
                "email": "Must be an email address.",
                "url": "Must be a URL.",
                "regex": "Invalid"
            },
            validatorFunctions: { // Return true if the value passes validation
                "required": function(el, value) { return ! isEmpty(value); },
                "min": function(el, value, args) { return isEmpty(value) || value.length >= args.length; },
                "max": function(el, value, args) { return isEmpty(value) || value.length <= args.length; },
                "accepted": function(el, value) { return el.is(":checked"); },
                "number": function(el, value) { return isEmpty(value) || (!isNaN(value) && !/^\s+$/.test(value)); },
                "alpha": function(el, value) { return isEmpty(value) || /^[a-zA-Z]+$/.test(value); },
                "date": function(el, value) { return isEmpty(value) || !isNaN(new Date(value)); },
                "email": function(el, value) { return isEmpty(value) || /\w{1,}[@][\w\-]{1,}([.]([\w\-]{1,})){1,3}$/.test(value); },
                "url": function(el, value) { return isEmpty(value) || /(((ht|f)tp(s?):\/\/)|(www\.[^ \[\]\(\)\n\r\t]+)|(([012]?[0-9]{1,2}\.){3}[012]?[0-9]{1,2})\/)([^ \[\]\(\),;&quot;'&lt;&gt;\n\r\t]+)([^\. \[\]\(\),;&quot;'&lt;&gt;\n\r\t])|(([012]?[0-9]{1,2}\.){3}[012]?[0-9]{1,2})/.test(value); },
                "regex": function(el, value, args) { return isEmpty(value) || args.expression.test(value); }
            }
        }, options);

        this.live(opts.validateOn, function() {
            var el = jQuery(this), validators = {};
            var validationString = expandValidatorString(el.data("validator") || "");
            try { validators = eval("({" + validationString + "})"); } catch (e) { } // Keeps invalid validation methods from raising errors
            var state = validate(el, validators, el.val());
            render(el, state);
            notify(el, state);
        });

        var expandValidatorString = function(str) {
            for (method in opts.validatorFunctions) {
                var containsExp = new RegExp(method);
                var testExp = new RegExp(method + "\\b(:|\\s:)");
                if (containsExp.test(str) && !testExp.test(str)) {
                    str = str.replace(containsExp, method + ":{}");
                }
            }
            return str;
        };

        var validate = function(el, validators, value) {
            var state = { isValid: true, errors: [] };
            for (method in opts.validatorFunctions) {
                if (!validators[method]) { continue; }
                var errOpts = validators[method];
                if (opts.validatorFunctions[method](el, value, errOpts)) { continue; }
                state.isValid = false;
                state.errors.push({
                    method: method,
                    message: errOpts.message ? errOpts.message: opts.defaultMessages[method],
                    title: errOpts.title ? errOpts.title: opts.defaultMessages[method]
                });
            }
            el.data("validation-state", state); // Adds the current validation state to the DOM element
            return state;
        };

        var render = function(el, state) {
            if (el.parent().is("." + opts.wrapperClass)) {
                jQuery("span." + opts.messageClass, el.parent()).remove();
                el.unwrap();
            }
            if (!state.isValid) {
                el.wrap(jQuery("<div class='" + opts.wrapperClass + "'></div>"));
                var message = jQuery("<span class='" + opts.messageClass + "'></span>");
                var text = "", title = "";
                for (var i = 0; i < state.errors.length; i++) {
                    if (state.errors[i].message && opts.useMessages) {
                        text = text + state.errors[i].message;
                    }
                    title = title + state.errors[i].title;
                }
                message.text(text);
                el.parent().append(message).attr("title", title);
            }
        };

        var notify = function(el, state) {
            var container = el.closest("form"); // Validation events will be triggered on the nearest form or body if no form is present
            if (container.length == 0) { container = jQuery("body"); }
            container.trigger("validation", [{ el: el, state: state }]);
            if (opts.onValidation) { opts.onValidation(el, state); }
            if (state.isValid) { return; }
            container.trigger("validationFailed", [{ el: el, state: state }]);
            if (opts.onValidationFailed) { opts.onValidationFailed(el, state); }
        };
    };

})(jQuery);

usage.html

<!DOCTYPE html>
<html>
	<head>
		<title>Usage Example for jQuery.validator</title>
		<script type="text/javascript" src="https://ajax.googleapis.com/ajax/libs/jquery/1.4.4/jquery.min.js"></script>
		<script type="text/javascript" src="jquery.validator.js"></script>
		<script type="text/javascript">
			$(function() {
				
				$(":input").validator();
				
				$("#form1").bind("validation", function(e, validation) { 
					alert("The field is: " + (validation.state.isValid ? "valid." : "invalid.")); 
				});
				
				$("#form2").bind("validationFailed", function(e, validation) {
					alert("Validation has failed :(");
				});
			});
		</script>
		<style type="text/css">
			
			/* Styles used by validator (can be modified via options) */
			div.invalid input[type='text'],
			div.invalid input[type='password'],
			div.invalid select {
				border: 1px solid red;
			}
			div.invalid span.message {
				color: red;
				font-weight: bold;
			}
			
			/* Demo styles */
			div.field input { border: 1px solid #999; padding: 3px 5px; }
			div.field { margin-bottom: 20px; }
			div.field label { display: block; }
		</style>
	</head>
	<body>
		
		<div class="field">
			<label>Required</label>
			<input type="text" name="field1" data-validator="required:{message:'!!!'}" value="Remove me" />
		</div>
		<div class="field">
			<label>Number</label>
			<input type="text" name="field2" data-validator="number:{message:'***'}" /> 
		</div>
		<div class="field">
			<label>Required, Number</label>
			<input type="text" name="field3" data-validator="required,number:{message:'Needs a number!'}" value="12345" />
		</div>
		<div class="field">
			<label>URL</label>
			<input type="text" name="field4" data-validator="url" value="http://www.google.com" />
		</div>
		<div class="field">
			<label>Email</label>
			<input type="text" name="field5" data-validator="email" value="jim.bob@hillbilly.net" />
		</div>
		<div class="field">
			<label>Min Length (3)</label>
			<input type="text" name="field6" data-validator="min:{length:3}" value="abcd" />
		</div>
		<div class="field">
			<label>Max Length (5)</label>
			<input type="text" name="field7" data-validator="max:{length:5}" value="abcd" />
		</div>
		<div class="field">
			<label>Empty Validator Attribute</label>
			<input type="text" name="field8" data-validator="" value="I'm empty" />
		</div>
		<div class="field">
			<label>Invalid Validator Attribute</label>
			<input type="text" name="field9" data-validator="invalidmethod" value="My method doesn't exist" />
		</div>
		
		<form id="form1">
			<div class="field">
				<label>Will Alert On Validation</label>
				<input type="text" name="field10" data-validator="required" value="I'm required" />
			</div>
		</form>
		
		<form id="form2">
			<div class="field">
				<label>Will Alert On Validation Failed</label>
				<input type="text" name="field11" data-validator="required" value="I'm required" />
			</div>
		</form>
		
	</body>
</html>