I've spent the last several months working on various personal hobby projects using AVRs, so I thought I'd write something that I could share with the community.
FunkOS is an RTOS written from the grounds up in C, with AVR as the primary target (although support for MSP430 is in there, too, with other targets pending).
Even though it just hit a first release, it's got most of the goodies you'd expect from a reasonably-capable RTOS:
-Fully preemptive kernel
-Synchronization and protection objects
-Task-monitoring watchdog module
-Scalable - you only build the features you need
-Open-source, no restrictive terms for commercial use.
I've also tried to make the source easy to read and understand, keeping the API as simple as possible (and well documented).
At the moment I'm developing using SimulatorV2 for 644 and 323 parts, but haven't tested on real hardware.
You can download the latest version from the project page at:
Feedback is always welcome - let me know what you think!