Validity is a powerful and flexible jQuery form validation for making sure -on the client-side- that form values are just as expected.
It is lightweight (9kb) and can perform simple to complex, dynamic and conditional validations.
The plugin also gives full control over how validation messages will appear which makes it possible to match the look and feel of your site.
There are various ready-to-use rules for e-mail, date, number, URL, time, phone, zip, etc.
There are lots of logical operators like: lessThanOrEqualTo, maxLength, nonHtml, sumMax and more to create any custom rules.