Originally Posted by gulliver
Aros. It is open source (unlike others this makes it future proof), runs multiple hardware architectures. Its features and maturity right now, are between OS4 and Morphos. But nothing prevents it going much further.
Actually AROS is behind OS4, MOS and OS3.9 in terms of features. There are fundamental pieces of it missing (eg. ReAction), so it's not good for much in practice.