RethinkDB is the first open-source, scalable JSON database built from the ground up for the realtime web. It inverts the traditional database architecture by exposing an exciting new access model – instead of polling for changes, the developer can tell RethinkDB to continuously push updated query results to applications in realtime. RethinkDB’s realtime push architecture dramatically reduces the time and effort necessary to build scalable realtime apps.

RethinkDB Horizon is an open-source developer platform for building realtime, scalable web apps. It is built on top of RethinkDB, and allows app developers to get started with building modern, engaging apps without writing any backend code.

Horizon consists of three components:

Horizon server: a middleware server that connects to/is built on top of RethinkDB, and exposes a simple API/protocol to front-end applications.
Horizon client: a JavaScript client library that wraps Horizon server’s protocol in a convenient API for front-end developers.
Horizon CLI: a command line tool, hz, aiding in scaffolding, development, and deployment.

Træfɪk is a modern HTTP reverse proxy and load balancer made to deploy microservices with ease. It supports several backends (Docker, Swarm, Mesos/Marathon, Kubernetes, Consul, Etcd, Zookeeper, BoltDB, Rest API, file…) to manage its configuration automatically and dynamically.

A compact, cross-browser solution for using the JavaScript Notifications API

nlp_compromise does NLP in the browser. It's a handy, and not overly-fancy tool for understanding, changing, and playing with english. We've also got a modest, though ambitious plugin ecosystem.

Waybackpack is a command-line tool that lets you download the entire Wayback Machine archive for a given URL.

Frisby is a REST API testing framework built on node.js and Jasmine that makes
testing API endpoints easy, fast, and fun. Read below for a quick overview, or
check out the API documentation.

Save requests and organize your APIs into Collections. Create and send any HTTP request using the awesome Postman request builder. Write your own test cases to validate response data, response times, and more! Collaborate in real-time over your team’s shared APIs. Make sure everyone is always on the same page. Test public APIs in one click using the Run in Postman button.

The coach helps you find performance problems on your web page. Think of the coach as a modern version of YSlow. The coach will give advice of how your page can be faster.

Looks good to me (LGTM) is the most popular comment on Github, perhaps. But LGTM is boring. So we spice it up with some images.

New Relic’s SaaS-based Application Performance Monitoring helps you build, deploy, and maintain great web software.
Also, server monitoring, and more.

OWASP Mutillidae II is a free, open source, deliberately vulnerable web-application providing a target for web-security enthusiest. Mutillidae can be installed on Linux and Windows using LAMP, WAMP, and XAMMP. It is pre-installed on SamuraiWTF, Rapid7 Metasploitable-2, and OWASP BWA. The existing version can be updated on these platforms. With dozens of vulns and hints to help the user; this is an easy-to-use web hacking environment designed for labs, security enthusiast, classrooms, CTF, and vulnerability assessment tool targets. Mutillidae has been used in graduate security courses, corporate web sec training courses, and as an "assess the assessor" target for vulnerability assessment software.

Vue.js is a library for building interactive web interfaces. It provides data-reactive components with a simple and flexible API. Core features include:

Dead simple, unobtrusive reactivity using plain JavaScript objects.
Component-oriented development style with tooling support
Lean and extensible core
Flexible transition effect system
Fast without the need for complex optimization

View raw output for critical events across servers. SeaLion gives you a graphical overview of server activity. View performance of system/custom metrics across multiple servers at a glance. The horizon charts makes it easy for you to identify patterns and find critical issues.

Event-driven, non-blocking I/O with PHP.

React is a low-level library for event-driven programming in PHP. At its core is an event loop, on top of which it provides low-level utilities, such as: Streams abstraction, async dns resolver, network client/server, http client/server, interaction with processes. Third-party libraries can use these components to create async network clients/servers and more.

WebSockets for PHP

Ratchet is a loosely coupled PHP library providing developers with tools to create real time, bi-directional applications between clients and servers over WebSockets. This is not your Grandfather's Internet.

Instant visual diffing with CSS blend modes!
Simply enter the sites you want to compare. Locally hosted addresses and files also work (so you can see how your local changes affect current pages or previous versions).

I built securityheaders.io after deploying security headers like CSP and HSTS to my own site. I wanted a quick and easy way to check if other sites were using these headers and I figured I'd turn it into a useful tool for everyone to use!
There are services out there that will analyse the HTTP response headers of other sites but I also wanted to add a rating system to the results. The HTTP response headers that this site analayses provide huge levels of protection and it's important that sites deploy them. Hopefully, by providing an easy mechanism to assess them, and further information on how to deploy missing headers, we can drive up the usage of security based headers across the web.

Open source HTML5 hybrid app framework for PhoneGap & Cordova
Mobile-optimized HTML5, CSS and JavaScript with Web components
Responsive layout, Material and Flat design

Passbolt is a free and open source password manager that allows team members to store and share credentials securely. Passbolt is free, open source and respectful of your privacy. It is also extensible thanks to its restful API. It's based on OpenPGP and has a Firefox extension.