Minix 3
In case anyone has missed it, Minix 3 is out. An extremely small OS with a kernel mode code below 4k lines of code. The goal with Minix 3 is to be usable as a serious system on resource-limited and embedded computers and for applications requiring high reliability.
Minix 1 and 2 where mainly intended to be used as teaching tools.
So go the Minix 3 site and download the CD image and have it try, it can be run as a Live CD and it's only 300 Megabytes big in compressed format.