RIOT OS is a modern operating system for the IoT.
RIOT OS aims to support all modern standards, such as IPv6, 6LoWPAN, BLE, LoRaWAN or IEEE 802.15.4.
It provides a great toolset in order to build modern IoT applications. RIOT OS is focused on constrained devices. It therefore targets a similar area as realtime operating systems, even though it is more focused on modern standards defined by the IETF, IEEE and W3C.