Top 4 extendable open source projects

Contemplate: Fast, extendable object-oriented and light-weight Template Engine for PHP, Python, Node.js, Browser and XPCOM/SDK JavaScript

Extendable calculator for the 21st Century ⚡

#ChallengeTime Front-End Toolkit with useful additional CSS and JS Scripts http://gmkhussain.github.io/frontend

Core server of the SEPIA Framework responsible for NLU, conversation, smart-service integration, user-accounts and more.