Awesome

nOS

Features:

• Preemptive or cooperative scheduling (depending on your configuration)
• Can be tickless for battery-powered application
• Binary and counting semaphores
• Mutexes with priority ceiling or priority inheritance
• Queues for thread-safe communication
• Flags for waiting on multiple events
• Memory blocks for dynamic memory allocation
• Software timers with callback and priority
• Software interrupts (signal) with callback and priority
• Real-time module compatible with UNIX timestamp
• Software alarms with callback
• No limits on number of nOS objects, except your available memory
• Tiny footprint (as low as 1KB ROM/few bytes of RAM)
• Fully configurable (ROM and RAM)
• Open source, royalty free
• Win32 and Linux simulator available

Documentation