Here's a javascript example:

<SCRIPT LANGUAGE="JavaScript">
<!-- Begin
function textCounter(field, countfield, maxlimit) {
if (field.value.length > maxlimit)
field.value = field.value.substring(0, maxlimit);
else
countfield.value = maxlimit - field.value.length;
}
// End -->
</script>

Description:(Limited to 150 characters)
<textarea wrap="virtual" name="Description" cols=40 rows=4 onKeyDown="textCounter(this.form.Description,this.form.remLen,150);" onKeyUp="textCounter(this.form.message,this.form.remLen,125);"><%if Description%><%Description%><%endif%></textarea>
<input readonly type=text name=remLen size=3 maxlength=3 value="150"> characters left