"Source Available" means that μC/OS-III is:

Free – You can download the full source code of µC/OS-III.

Free – Accredited colleges and universities can use µC/OS-III without a license, as long as the software is used only for educational purposes or peaceful research.

Free – You can evaluate the µC/OS-III source code for up to 45 days for use in a commercial application.

Licensed – You need to purchase a license once you decide that you will be using µC/OS-III in your project(s). See our licensing information.

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제가 자주 가는 사이트에 올라와 있길래 스크랩 해놓습니다.
이전에도 무료였는지는 모르겠지만서도... 이쪽은 잠깐 밖에 안봐서. ^^;;

해당 주소는 http://micrium.com/page/downloads/os-iii_projects 네요. ^^