Available for GNU/Linux and FreeBSD (as of 0.0.3), and MS Windows (as of 0.5). With some sleight of hand, it is possible to get it running on other systems as well...
Requires SDL, zlib, bison and flex to build properly.
If you want access to the very state of the art untested sources, anonymous CVS access is available via the tunguska's sourceforge page.
Besides the code for a small "operating system" / standard library that ships with tunguska in the memory_image subdirectory of the package (read memory_image/README for more information), here are some files that can be assembled into images that can be loaded with the virtual floppy disk system, using the LOADSUB command at the prompt. See the quick guide in the manual for how to do it.