There are several options but Puma seems to be the most popular at the moment. A derivative of Unicorn named Pitchfork is also in the works from Shopify - http

There are several options but Puma seems to be the most popular at the moment. A derivative of Unicorn named Pitchfork is also in the works from Shopify - https://github.com/Shopify/pitchfork https://github.com/Shopify/pitchfork