jQuery.fn.center = function() {
	return this.each(function() {
		$(this).css("left",(($(document).width() / 2) - ($(this).width() / 2)) + "px").css("top",(($(window).height() / 2) - ($(this).height() / 2)) + "px");
	});
}

jQuery.fn.centerOnObject = function(obj) {
    return this.each(function(i) {
        $(this).css("left",obj.position().left + (obj.width()/2) - ($(this).outerWidth()/2) + "px");
        $(this).css("top",obj.position().top + (obj.height()/2) - ($(this).outerHeight()/2) + "px");
    });
}