If you¡¯re into the whole separation of behavior and structure thing, you know that unobtrusive JavaScript is the way to go. Chris goes over three different methods of validating your forms without mucking up your markup.
Complete listing of the Form object of JavaScript and its properties/methods. Also includes additional pages describing objects beneath Form, such as TEXTAREA, SELECT etc.
Sometimes you want to limit characters in a text box. If that is the case, try this script. This version limits the characters and tells you that you're off the limit with a sound of your choice.
For various web scripts it is more convenient that user will see form executed in new window when he is pressing the submit button. Using JavaScript you can control the appearance of that window. change size, turn off navigation panel, hide manubar and so on. This technique is very helpful for poll and guestbook applications.
Got a submissions form that you keep getting multiple submissions from? I've found that in most cases its because people are impatient and click the submit button repeatedly until they get a response. The unfortunate result is that you receive multiple copies of the same form. This little bit of JavaScript disables the submit button as soon as it is clicked for the first time and prevents the user sending their form multiple times. Script now also changes the text on the button to 'Thanks' (this can be modified to suit)
In forms when using text boxes or text areas with limited character length (usually needed for forms that submit data to a database) it is always a good idea to tell the user how many characters they have remaining. The following example shows how you can do this. This is a very simple and cute idea to help the user know exactly how many characters he can type further. Do these small add-ons to your forms and they will look really professional.