DIY things from

There are a lot of cool things.
How about DIY ZigBee CO2 sensor for less than $50? Датчик CO2 Zigbee | Всё чем хочется поделиться…
I know that the website is in Russian, but google translate could help.
What do you think about ground humidity sensor for flowers? Сенсор влажности почвы Zigbee. Версия №2 | Всё чем хочется поделиться…
And so on and so on.
Open source firmware, gerber and BOM files for almost every device to make it by yourself.