I’m honestly surprised it took this long, but WordPress.com has introduced a Portfolio custom content type for their platform.
Current themes that support the new content type are Espied and Illustratr, however I think that many themes will begin introducing support for it. This is really a much better practice than using a category of blog posts for portfolios, which is what WordPress.com did previously.
This isn’t the first custom post type WordPress.com has supported, but it is quite rare. Others supported by WordPress.com are custom food menus, testimonials, and comics. Those same three are also available to be supported in Jetpack, and I presume Portfolio will make its way to Jetpack as well.
You can see more about how Portfolios work on the support page.