The Validation Library is very customizable and you can also precisely say how do you want the error messages to be displayed like Scripting Alert Popup, inline DHTML Message (similar to ASP.NET RequiredFieldValidator and ValidationSummary for Microsoft Internet Explorer and uplevel browsers) etc.
Since this library is also a freeware downloadable and quite easy to configure, I hope it would be very useful for development fraternity of web-based applications worldwide.