How to get ID of button user just clicked?

With pure javascript:

var buttons = document.getElementsByTagName("button");
var buttonsCount = buttons.length;
for (var i = 0; i <= buttonsCount; i += 1) {
    buttons[i].onclick = function(e) {
        alert(this.id);
    };
}​

http://jsfiddle.net/TKKBV/2/

$("button").click(function() {
    alert(this.id); // or alert($(this).attr('id'));
});