Back in 2003, United-TI formed from the merge of various websites such as Cirrus TI Programming and The Calc Site and various projects were announced. Among them there were Contra 83, Sonic, NimbusOS and plenty of others. Unfortunately, very few were finished. Recently, however, David, one of the founders of United-TI, has published the source and executable of NimbusOS as is.
As this screenshot shows, this is a calculator shell. As the
United-TI post states, this shell has bugs, though, so
use it at your own risk! It is released as it is as the authors are officially no longer working on it. Although it is not recommended for your gaming needs due to its uncertain stability, maybe you will want to see, out of curiosity, what alternative to other shells was in development back in the days. Maybe the source code will eventually be useful to other programmers as well.
More information as well as some download links are available
here.