$("#textarea").keyup(function(){
$("#count").text($(this).val().length);
});
The above will do what you want. If you want to do a count down then change it to this:
$("#textarea").keyup(function(){
$("#count").text("Characters left: " + (500 - $(this).val().length));
});
Alternatively, you can accomplish the same thing without jQuery
using the following code. (Thanks @Niet)
document.getElementById('textarea').onkeyup = function () {
document.getElementById('count').innerHTML = "Characters left: " + (500 - this.value.length);
};