Opa is described as 'open source, simple and unified platform for writing web applications. All aspects are directly written in Opa: Frontend code, backend code, database queries and configuration. And everything is strongly statically typed'. There are more than 25 alternatives to Opa for a variety of platforms, including Linux, Mac, Windows, Online / Web-based and Self-Hosted solutions. The best alternative is Node.js, which is both free and Open Source. Other great apps like Opa are AngularJS (Free, Open Source), ExpressJS (Free, Open Source), Meteor (Free, Open Source) and Polymer (Free, Open Source).
Polymer is a library that uses the latest web technologies to let you create custom HTML elements. Build anything from a button to a complete application as an encapsulated, reusable element that works across desktop and mobile.
Forward-thinking Written with ES2016. Integrates with Web Components. No external dependencies except polyfills. Leverage the technology of the future but target today's mobile, desktop and browser environments.