VIA article from Peter Blum on ASPaaliance
Validating user input is an important requirement of most web applications. However, many ASP.NET developers find the standard ASP.NET validation controls to be either hard to use or lacking in features. Peter Blum answers the most frequently asked questions concerning ASP.NET validation.
Some of the topics he discussed are:
(1) Client-side validation does not work at all
(2) Buttons do not submit
(3) When to Validate
(4) Preventing a button from validating
(5) Several buttons associated with their own validators
(6) AutoPostBack and validation
(7) How to display MessageBoxes on Validation errors.
(8) Validators support for globalization
(9) Using validators with DropDownLists and ListBoxes
(10) Making the CustomValidator support blank textboxes
Here is the article Solving the Challenges of ASP.NET Validation