What errors are you seeing in the browser? I can understand why your code doesn't work if what you posted was incomplete, but without knowing that I can't know for sure.
<!DOCTYPE html>
<html>
<head>
<script src="http://code.jquery.com/jquery-1.5.js"></script>
<script>
function countChar(val) {
var len = val.value.length;
if (len >= 500) {
val.value = val.value.substring(0, 500);
} else {
$('#charNum').text(500 - len);
}
};
</script>
</head>
<body>
<textarea id="field" onkeyup="countChar(this)"></textarea>
<div id="charNum"></div>
</body>
</html>
... works fine for me.
Edit: You should probably clear the charNum div, or write something, if they are over the limit.
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