Text

• Etherpad - Collaborative real-time editing

Spreadsheets

• EtherCalc - Collaborative real-time spreadsheet

Web

• Hotglue - Collage-like WYSIWYG editing for text, images, videos

Graphics

• SVG-edit - Fast vector graphics editing
• scri.ch - Simple bitmap drawing
• Prototypo - Font editing

3D

• three.js editor - WebGL-based geometry editing
• clmtrackr - Webcam face substitution
• headtrackr - Webcam head tracking
• js-aruco - Augmented reality using webcam and printed markers

Programming

• p5.js editor - Processing-like environment for the Web (unclear if the editor will also run inside a browser window, but the library itself is pretty straightforward to use with CodePen or similar)
• paper.js - The JavaScript-based vector graphics framework has an online code editor
• VVVV.js - Visual, flow-graph programming
• CodeMirror - Text editor for code (used by JSFiddle and others)
• GitLab - Manage multiple Git repositories (à la GitHub)

Data visualization

• Tributary - Rapid prototyping with d3.js

Audio synthesis

• WebPd - PureData flow-graph audio synthesis language on the web
• Flocking
• timbre.js
• Gibber
• CoffeeCollider

Other

• FramaDate - Online polls, also for scheduling events, based on Studs
• Kanboard - Project management using the Kanban method
• mindmaps - Draw mindmaps in the browser
• uMap - Create embeddable maps based on OpenStreetMaps
• Lufi - Encrypted file upload & download
• ZeroBin - Encrypted text upload (à la Pastebin)
• Lutim - Image upload for embedding (à la Imgur etc)

Social

• GNU Social - Federated social networking (à la twitter), used by Richard Stallman himself
• GNU FM - Federated music community platform (à la Last.fm)