9vx was originally written by rsc. A version of it is included in the vx32 distribution. Users can run that version against a full Plan 9 tree to get a very good emulation of Plan 9.
Note that since the entire system runs as the invoking user, some things aren't quite right. For example, all file access is done as the invoking user, not the Plan 9 user. Networking is also provided by a stripped-down IP stack that passes through to the host OS's IP stack, but only tcp and udp.
There are extensions to 9vx to provide it a raw ethernet device (which it may then run a real Plan 9 IP stack above) and allow direct access to raw disk devices, which allow it to use a real fossil (and optionally venti) partition, instead of #Z. Search 9fans; Brian Stuart did the later; dho and/or quanstro did the networking. When you find good links, come back and put them here!
For more information, see Russ's original 9vx page.
Several users have modified and extended 9vx to their needs. See the patches for their details.