HelenOS (http://www.helenos.org/) and Minix (http://www.minix3.org/) are fundamentally different. While Minix is POSIX compliant, HelenOS has been designed and developed from scratch, improving upon some of the broken legacy conventions in the way. It is not even POSIX compliant, though it has libposix library for now to ease the task of porting foreign sources!

HelenOS uses its own microkernel written from scratch and supports SMP, multitasking and multithreading on both 32-bit and 64-bit, little-endian and big-endian processor architectures, among which are AMD64/EM64T (x86-64), ARM, IA-32, IA-64 (Itanium), 32-bit MIPS, 32-bit PowerPC and SPARC V9.

HelenOS is infact one of the most portable microkernel operating system! Currently it's not as much developed as it's counterparts but it will become a complete and usable modern operating system very soon given the amount of activity going on in there.