Webtask!

Cloud9 is a code editor and development environment built with browser technology yet with the power of a desktop IDE. It's available both as an online service and as a locally hosted platform.

Heroku is a cloud application platform for Ruby, Rails, Node.js, Java, Python, Scala and Clojure web apps. Everything you need to build, run, and scale:

Google App Engine is a free* platform for developing and hosting web applications in Google-managed data centers. App Engine offers fast development and deployment; simple administration, with no need to worry about hardware, patches or backups; and effortless scalability.

Codeanywhere is a code editor in a browser with a powerful ftp client integrated, and all popular web formats are supported (HTML, PHP, JavaScript, CSS, and XML).

PythonAnywhere is a Python development and hosting environment that displays in your web browser and runs on cloud servers. The servers are already set up with everything you need: hundreds of useful Python packages and web frameworks are supported out of the box.

Extend Google Apps and build web applications with this JavaScript cloud scripting language stored in Google Drive and run from Google's servers.

AWS Lambda is a compute service that runs your code in response to events and automatically manages the underlying compute resources for you. You can use AWS Lambda to extend other AWS services with custom logic, or create your own back-end services that operate at AWS scale, performance, and security. AWS Lambda can automatically run code in response to modifications to objects in Amazon S3 buckets, notifications sent from Amazon SNS, messages arriving in Amazon Kinesis streams, or table updates in Amazon DynamoDB. »

With working example apps to remix, a code editor to modify them, instant hosting and deployment - anybody can build a web app on Glitch.