Arduino IDE - Arduino is an open-source electronics prototyping platform based on flexible, easy-to-use hardware...
PlatformIO - Next-generation IDE for IoT
B4R - Development tool for native Arduino programs.
openHAB - "empowering the smart home" - vendor and technology agnostic open source home automation
embedXcode - embedXcode is a template for Xcode.
Zerynth - Zerynth speeds up IoT development. With Zerynth you can program in Python or hybrid C/Python language the most popular 32-bit microcontrollers, and connect them to the top Cloud infrastructures.