A few years there was some efforts done to port AROS (API compatible to Amiga OS, open source, available on different platforms) native to RPi. That was stopped finally (one reason I think was USB support could not be done), now one developer again is working on it.
He posts his progress and it is a very interesting read:
The general AROS forum is here:
And more informations including AROS nightlies (bare test versions of AROS):
For X86 there are two end-user distributions available: