Ruven

This plugin registers a custom post type for portfolio items and a taxonomy for portfolio types.
Selected themes by Ruven provide templates to display portfolio items. 
In other themes you most likely have to add them by yourself.

Ruven Themes: Portfolio

RT Portfolio enables a "Portfolio" post type and "Type" taxonomy.

alexmustin

thebrandid

Seeing is believing. And one of the strongest ways to communicate your expertise is with a visual portfolio of bona fides scrolling along your website. With each click, a slide appears sharing testimonials, logos of awards, reviews, sites you’ve appeared in or been featured on, or links to your guest posts on popular blogs. It’s a quick, seamless way to communicate your online authority, credibility, and skill set through social proof. And it’s easy to populate, update, and customize on your WordPress site. As the Social Proof Slider scrolls, your authority grows.

Social Proof (Testimonial) Slider

Showcase your expertise with an easy-to-customize responsive carousel slider of your featured testimonials.

Ruven Themes: Portfolio vs Social Proof (Testimonial) Slider