Circuito.io - Instant code and wiring for your project

MINIBLOQ - miniBloq is an open source graphical programming environment for Multiplo™, Arduino™, physical computing devices and robots.

library.io - Learn electronics and easily create circuit boards online

Modkit - Modkit is an open-source programming platform that allows programmers with a variety of backgrounds to share their creations and learn from each other and is allowing programmers to make beautiful software.

B4R - Development tool for native Arduino programs.

Visuino - Visuino is a visual programming environment that lets you write programs from Arduino circuit boards.