Hmm. I like how someone mentions Unix and everyone assumes Linux.
I suggested Minix for a number of reasons. Firstly, the licence is compatible, but secondly, the Minix kernel is technically far superior to Linux - it is a proper microkernel. The Linux kernel is historically a large monolithic kernel, with some drivers considered part of it. Drivers run in supervisor mode, so offer little protection. Things are changing, but the underlying design is not neat.
The downside is that Minix has very few drivers, and doesn't even support the ARM yet...