Better Approach:
<script type="text/javascript">
$('#btn').click(function() {
var id = $(this).attr('id');
alert(id);
});
</script>
<input id="btn" type="button" value="click" />
But, if you REALLY need to do the click handler inline, this will work:
<script type="text/javascript">
function display(el) {
var id = $(el).attr('id');
alert(id);
}
</script>
<input id="btn" type="button" value="click" OnClick="display(this);" />