Some of the recent discussions on this forum regarding "Tickless OS" support NuttX inspired me. So I implemented it! It really was not that difficult... About a day and a half of work with only a few spills and chills.
I did all of the testing on the NuttX simulator using the OS test. I built in a simulated interval timer for the simulator and ran the OS test against the tickless OS support. The OS test is probably more exhaustive than what the typical application does so I am fairly confident in the implementation. Of course the simulator can miss certain classes of bugs.
NuttX is a 32bit embedded realtime operating system, licensed as open source under the BSD license.