+7  A: 
GerManson
i found also http://livevalidation.com/ and http://www.geektantra.com/2009/09/jquery-live-form-validation/ but do u thing that 'jQuery Validator' is the best ?
Space Cracker
is quite popular, you will find a lot of documentation and help around. and its really easy to use and to extend,
GerManson
Agreed. But it won't do validation on server side with SQL Server.
Ismail
+1  A: 

Before you go into best user experience in form validation you have to put some consideration while implementing it,
the validation must always be in the server side and never go only in client side validation in case the user disables javascript or he can change the validation expression using some tools, therefor there is two ways you do it.
ajax validation or multiple validation
in Ajax validation when the user focus out the item it directly posts the data to server validates it and then you put the UI and do validation on submit too.
or you make validation in javascript for the UI without AJAX and then you do other validation in server side on submit. it means you need to copy the validation expressions once in server and once in javascript.

after you decide which method you go then you find the most appropriate tool for it. and I recommend you to go with jQuery plugins they are very easy to use

Kronass
thanks for these considerations and are really very important
Space Cracker
A: 

ASP.NET Server Validation Controls in conjunction with ValidatorCallout from Ajax Control Toolkit is also a good option. It can do validations on both server and client end. Obviously it will require more effort than jQuery.

Ismail