Ketchup is an easy-to-customize (in means of appearance & functionality) jQuery plugin for validating user inputs on the client-side.
The plugin works by inserting the validation rules inside the markup, just like:
<input type="password" class="validate" id="new_pass"
It comes with a bunch of ready-to-use rules, including: required, min/max length, number, URL, e-mail & much more where new ones can be created as well.
Using Ketchup, it is possible to combine validations (a field can be both required & e-mail).
Although the default setup of the plugin is very satisfactory, like mentioned, it is possible to create a new look & feel (which is easy with the documentation provided).