It's a bit OT, but allow me to plug my FreeRTOS port (NOT using the online compiler, rather using gcc: http://github.com/hugovincent/mbed-freertos)... I did a lot of work on the toolchain and build system so that it can aggressively optimize out any parts of the operating system you aren't using in your application. RAM usage is ambiguous as FreeRTOS malloc()s some important memory, but a typical hello world app uses a similar amount of RAM as the CoOS example above and around 30-45 kB of flash, while a more complete app, with a TCP/IP stack, ethernet and DMA-driven buffered serial ports, mbed file system access, protected memory (using the MPU), debug handlers and so on, will use 75-85 kB of flash (edit: this is the total binary size, including all libraries etc and wastage necessitated by alignment for the MPU). Using my port, you will also get* thread-safe C/C++ libraries, which is impossible using the online compiler (from what I can see anyway... correct me if I'm wrong).
From what I've seen, FreeRTOS is a more mature, robust and complete RTOS compared to CoOS. However, using the online compiler might be seen as a big advantage, and it's going to be pretty difficult to get a good FreeRTOS port working using the online compiler**.
* OK they're not 100% complete yet, but something is better than nothing, right :-)
** Reason: for the protected memory/MPU stuff to work, we needed to make substantial changes to the linker scripts and C libraries... which we can't do with the online compiler.
Nice work!