Writing your own custom validation rules is sometimes necessary, but it can also be really time consuming. Any of the following lightweight plugins will do most of the validation code writing for you.
quickValidation allows you to define the validation rules directly within the HTML input tags. You can also use this plugin to insert custom some custom error text.
Ketchup comes with 18 validations, but you can easily expand on these by writing your own (or tweaking the existing ones). The plugin is also really lightweight.
This client side validation plugin has many validations methods included with the install, but as with the Ketchup plugin, it’s pretty easy to write your own.
Validatr uses HTML input attributes to perform validation and will use native validation wherever possible.
This plugin is a lot like the ones listed above, except it comes with a little bit of smooth animation. When an input item is focused on, the label will slide to the left of the input, and then the label will fade out after a value is entered.