Did you ever found a need where you want to prevent your user from entering certain characters?
Looking at the plugins available at jQuery, i found a great plugin by Sam Collet called Numeric.
But it was too limited, what if I'm asking the user to create a username? or 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. but the problem was the length of text to be input 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.