AROS: Amiga Research OS  
   Goal: create an OS as compatible as possible to AmigaOS 3.1, but which can be ported to various CPUs: x86, 68k; PowerPC, Alpha, SPARC, MIPS.
   http://www.aros.org/

NetBSD/amiga - Un*x for the amiga  
   NetBSD/amiga is a free, portable unix like operating system for many machines, including the Amiga.
   http://www.netbsd.org/Ports/amiga/

OpenBSD/amiga  
   OpenBSD port to Amiga: the same as the NetBSD/amiga port but with some additions.
   http://www.openbsd.com/amiga.html

NetBSD 1.3 for Amiga  
   NetBSD port for Amigas with MMUs.
   http://www.netbsd.org/Releases/formal-1.3/amiga.html

MorphOS  
   Goal: provide modern functionality for a new OS layer and run most Amiga applications as efficiently as possible in a box environment with no old Amiga custom chips available.
   http://www.morphos.net/

Multi User System Security  
   A low level security system for the Amiga, consisting of a new filesystem and a library. It implements a security system similar to that of UNIX.
   http://mufs.sourceforge.net/