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). (link is dead)
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.")