os - What Operating Systems are available for Arduino?

This question seems to suggest that people are developing Operating Systems for the Arduino platform.

• What Arduino OS projects are you aware of?

Please list only one project or OS per answer, and give a short list of goals or features of that OS.

For the purposes of this question software frameworks, task handling utilities, and similar projects are considered Operating Systems. While there are many AVR frameworks and OS projects that work on Arduino hardware, please only list those that work within the Arduino software development tools and platform without modification.

Answer

Here is a good run-down of four different OS for the Arduino http://antipastohw.blogspot.com/2009/11/4-operating-systems-for-arduino.html