Runs in ACL 5.0 but "intended to be very portable".
Also, while ACL is the "prefered" Lisp dialect, most of the code is pretty vanilla CommonLisp/CLOS, and the ACL-dependent parts (dealing primarily with wrappers around ACL's socket-implementation and the definition of some numeric variables) are neatly packed away in a couple files in a sub-directory (that is: I've not really had time to work on ports to other lisp-implementations, but my intent is that they would be pretty painless.)
The main page is here. The executable code, documentation and examples for this pre-release (v1) version of the LIJOS system, for use with Allegro CommonLisp (v5.0) can be found at: http://www.pointnclick.com/lijos/index.html (Windows and Sparc).
The license is pretty liberal ("Permission to use, copy, and distribute this software is hereby granted, provided that this copyright notice is retained in all copies.")