Did you ever have the need to prevent users from entering certain characters into your form?
Looking at the plugins available at jQuery, I found a great plugin made by Sam Collet called Numeric.
But it was too limited, what if I'm asking the user to create a username? Or what if I need to enter a number with decimals or an IP Address? There is another great plugin called Masked Input by Josh Bush, which can also control user input by defining a mask. The problem however with that was the length of text to be inserted must be defined as well. Again, what if I needed to control input of a username? I can't tell how many characters the user will be using, and I can't force him to use just 8 characters ,so I created AlphaNumeric.
jQuery AlphaNumeric is a javascript control plugin that allows you to limit what characters a user can enter on textboxes or textareas. Have fun.