Blockly 
Google's Blockly is a web-based, visual programming editor. Users can drag
blocks together to build programs. All code is free and open source.
The project page is https://developers.google.com/blockly/
Blockly has an active
developer forum. Please drop by and say hello. Show us your prototypes early; collectively we have a lot of experience and can offer hints which will save you time.
Help us focus our development efforts by telling us
what you are doing with
Blockly. The questionnaire only takes
a few minutes and will help us better support the Blockly community.
Want to contribute? Great! First, read
our guidelines for contributors.