var $local_source = ["c++", "java", "php", "coldfusion", "javascript", "asp", "ruby"];
$("#txtAllowSearch").autocomplete({
    source: $local_source
});

$('#button').click(function() {
    // alert($("#txtAllowSearch").someone_get_id_of_selected_item);
});

$('#selector').autocomplete({
    source: url,
    select: function (event, ui) {
        $("#txtAllowSearch").val(ui.item.label); // display the selected text
        $("#txtAllowSearchID").val(ui.item.value); // save selected id to hidden input
    }
});

$('#button').click(function() {
    alert($("#txtAllowSearchID").val()); // get the id from the hidden input
});

var $local_source = [[0,"c++"], [1,"java"], [2,"php"], [3,"coldfusion"],
                     [4,"javascript"], [5,"asp"], [6,"ruby"]];

var $local_source = [{
    value: 1,
    label: "c++"
}, {
    value: 2,
    label: "java"
}, {
    value: 3,
    label: "php"
}, {
    value: 4,
    label: "coldfusion"
}, {
    value: 5,
    label: "javascript"
}, {
    value: 6,
    label: "asp"
}, {
    value: 7,
    label: "ruby"
}];

$('#sistema_select').autocomplete({

    minLength: 3,
    source: <?php echo $lista_sistemas;?> ,
    select: function (event, ui) {
         $('#sistema_select').val(ui.item.label); // display the selected text
         $('#sistema_select_id').val(ui.item.value); // save selected id to hidden input
         return false;
     },
    change: function( event, ui ) {
        $( "#sistema_select_id" ).val( ui.item? ui.item.value : 0 );
    }
});

var $local_source = [
       {value: 1,  label: "c++"},
       {value: 2,  label: "java"}]

{"list":[
     {"value": 1, "label": "abc"},
     {"value": 2, "label": "def"},
     {"value": 3, "label": "ghi"}
    ]}

source: function(request, response) {
        $.getJSON("listing.php", {
            term: request.term
        }, function(data) {
            var array = data.error ? [] : $.map(data.list, function(m) {
                return {
                    label: m.label,
                    value: m.value
                };
            });
            response(array);
        });
    },
    select: function (event, ui) {
        $("#autocomplete_field").val(ui.item.label); // display the selected text
        $("#field_id").val(ui.item.value); // save selected id to hidden input
        return false;
    }

<script type="text/javascript">
$(function () {
    $("#MyTextBox").autocomplete({
        source: "MyDataFactory.ashx",
        minLength: 2,
        select: function (event, ui) {
            $('#MyIdTextBox').val(ui.item.id);
            return ui.item.label;
        }
    });
});