Magento2 How to show sku in order summary?

Override one template (simplification of Joey's answer). Bind directly to window object.checkoutConfig.quoteItemData object.

app/design/frontend/MyCustomTheme/default/Magento_Checkout/web/template/summary/item/details.html

<!-- ko foreach: window.checkoutConfig.quoteItemData -->
    <!-- ko if: item_id == $parents[1].item_id -->
        <span class="label" data-bind="i18n: 'SKU:'"></span>
        <!-- ko text: sku --><!-- /ko -->
    <!-- /ko -->
<!-- /ko -->

"===" cant be used in item_id == $parents[1].item_id so we use "=="

In magento 2, you can display sku without creating any plugins, I'm just posting this answer to help out someone if needed..

Override

vendor/magento/module-checkout/view/frontend/web/template/su‌​mmary/item/details.h‌​tml file or find that relevant file inside your theme. You should have Magento_Checkout inside your theme folder.

Add

<strong class="product-item-sku" data-bind="text: getSku($parent.item_id)"></strong>,

wherever you want.

Then override vendor/magento/module-checkout/view/frontend/web/js/view/sum‌​mary/item/details.js

file or find that relevant file inside your theme. You should have Magento_Checkout inside your theme folder. Then add getSku function to it.

your details.js file should looks like this with added getSku function,

/*** Copyright © 2013-2017 Magento, Inc. All rights reserved.
* See COPYING.txt for license details.*//*jshint browser:true
 jquery:true*//*global alert*/

define(['uiComponent'], function(Component) {
    "use strict";
    return Component.extend({
        defaults: {
            template: 'Magento_Checkout/summary/item/details'
        },
        getValue:

            function(quoteItem) {
                return quoteItem.name;
            },
        getSku: function(itemId) {
            var itemsData = window.checkoutConfig.quoteItemData;
            var prodSku = null;
            itemsData.forEach(function(item) {
                if (item.item_id == itemId) {
                    prodSku = item.sku;
                }
            });
            if (prodSku != null) {
                return 'SKU: ' + prodSku;
            } else {
                return '';
            }
        }
    });
});

Thanks Milan Pattani for his huge help