GeneraGenera is the OS that runs on Symbolics Lisp Machines; it also runs under hardware emulators for Mac, and under a software emulator on 64-bit Alpha machines.
To answer an obvious question, the software emulator only runs on 64-bit processors because the tagged-memory architecture of Symbolics machines used 40-bit words with 36 of those bits for actual data. Emulating this on a common 32-bit machine is prohibitive to say the least.
The product is not actively developed anymore, but is still serviced, and it still does amazing things that have been lost since the times of Lisp machines. Being a Lisp-based open system, it's an example of a no-kernel system. It also has a tightly-integrated system of interface, first developed as Dynamic Windows which later developed into CLIM. The system is still used for Common Lisp and other language development in modern times simply for the development environment and intelligent interface systems.
Some significant features of it which are non-obvious to outsiders include versioning built into the file system, support for many different programming languages, support for several different memory-allocation schemes (using memory-region allocation and referencing), and a low-level version of Lisp for controlling the boot sequence and configurations of hardware.
The Mac-Version is not a hardware emulator. It is a real Ivory-processor on a real Lisp machine Nubus board.