Preact!

Vue.js - Intuitive, Fast and Composable MVVM for building interactive interfaces.

Gem.js gives you a way to create modular, composable web components and modular, composable styles in pure-javascript. It also includes a set of extensible web components for building visual user interfaces and styling web applications.

Riot lets you build user interfaces with custom tags using simple and enjoyable syntax. It uses a virtual DOM similar to React but faster. Riot is very tiny compared to industry standards. We think there is a clear need for another UI library.

MWF works together as a series of page types, modules, and components. It’s a system that allows site authors to consider how best to tell the story of Microsoft products and services with confidence that the modular pieces will support and work together.

Marko is a modern, progressive UI library built and used by eBay for fast and friendly web development. It provides an elegant and readable syntax for both single-file components and components broken into separate files and includes a bunch of examples and an intuitive tool to try it out online.

Rax is a universal JavaScript library built and used by Alibaba.com that has a largely React compatible API.