Okamoto Hidetaka

This plugin gives you Related Item Widget and function.
If you want to show Related Item List, you can do following actions:
<ul>
<li>Add <code>Elasticommerce Related Widgets</code> in your widgets area.</li>
<li>Add <code>escr_related_item();</code> function in your theme or plugins.</li>
<li>If you only want to get the data, you can use <code>escr_get_related_item();</code> function.</li>
</ul>
Notice 
This plugin requires WooCommerce Plugin , so please install it. If WooCommerce Plugin is not installed or activated, this plugin will not work.
Required 
This Plugin cannot use Elasticsearch version 2.x. Please use Elasticsearch version 1.5 instead.

Elasticommerce Related Items

You can get good Related Item List powered by Elasticsearch.

Nima Shayanfar

Bushwack Studio

ElasticProbe, a fast and flexible search and query engine for WordPress, enables WordPress to find or “query” relevant content extremely fast through a variety of highly customizable features. WordPress out-of-the-box struggles to analyze content relevancy and can be very slow. ElasticProbe supercharges your WordPress website making for more sales, happier users and administrators. The plugin even contains many features.
Here is a list of the amazing ElasticProbe features included in the plugin:
Search: Instantly find the content you’re looking for. Even when you misspell.
Instant Results: A built for WordPress search experience that bypasses WordPress for optimal performance. Instant Results routes search queries through a dedicated API, separate from WordPress which results in lightning fast search experience.
WooCommerce: With ElasticProbe, filtering WooCommerce product results is fast and easy. Your customers can find and buy exactly what they’re looking for, even if you have a large or complex product catalog.
Related Posts: ElasticProbe understands data in real time, so it can instantly deliver engaging and precise related content with no impact on site performance.
Protected Content: Optionally index all of your content, including private and unpublished content, to speed up searches and queries in places like the administrative dashboard.
Documents: Indexes text inside of popular file types, and adds those files types to search results.
Autosuggest: Make (post, page or custom type) suggestion while user is typing in the search input.
Autocomplete: Complete the search query term while the user is typing.
Filters: Add controls to your website to filter content by one or more taxonomies.
Comments: Indexes your comments and provides a widget with type-ahead search functionality. It works with WooCommerce product reviews out-of-the-box.

ElasticProbe

A lightning fast and flexible search and query engine for WordPress.

Elasticommerce Related Items vs ElasticProbe