Wikipedia
LiteOS is a real-time operating system (RTOS) from University of Illinois for use in sensor networks. LiteOS is a Unix-like operating system that fits on memory-constrained sensor nodes. This operating system allows users to operate wireless sensor networks like operating Unix, which is easier for people with adequate Unix background. LiteOS provides a familiar programming environment based on Unix, threads, and C. It follows a hybrid programming model that allows both event-driven and thread-driven programming. LiteOS is open source, written in C and runs on the Atmel AVR based MicaZ and IRIS sensor networking platform.
The latest version of LiteOS is 2.0.1, which was released on September 12, 2011. According to the official release notes, this version is closely integrated with AVR Studio 5.0. With the JTAG support which is natively built into this software, LiteOS allows debugging sensor network applications directly through JTAG and AVR Studio.