Powerful & Simple Javascript Forms Validation Library: Parsley.js
Parsleyjs is a highly configurable and customizeable javascript library that make forms validation very simple and easy. It has a very powerful DOM API that validate the form even without writing a single line of code. You can overide every default behaviour of parselyjs to fit the needs of your own needs. It works with jQuery and Zepto also.
Parselyjs provides the default validation of many common scanrios such as email,url, minlength, maxlength,digits, date etc. It also provide remote valdation easy. you can use remote method to validate weather your required field is valid or not. The library also provide support to add custom validator according to your requirements.
The error messages of parsleyjs can be customized easily with little css haeadce. It is a well documented and also support localization.