NodeMCU. Connect Things EASY. An open-source firmware and development kit that helps you to prototype your IOT product within a few Lua script lines. Features:

• Open-source,
• Interactive,
• Programmable,
• Low cost,
• Simple,
• Smart,
• WI-FI enabled

The module is for sale in AliExpress website, and cost around € 9,40 the unit.

An example project for room monitoring can be found here.

Information

• Website: http://nodemcu.com/index_en.html
• GitHub: https://github.com/nodemcu
• Wiki: https://github.com/nodemcu/nodemcu-firmware/wiki
• Community: http://bbs.nodemcu.com/