Worlds of Ultima: Savage Empire was released in 1990, and used the same engine as Ultima 6 (albeit with a different graphics tileset). It is not considered a canon Ultima.

Savage Empire saw Lord British sending the Avatar to the land of Eodon, a prehistoric jungle empire, to defeat the Myrmidex queen and shatter a dark moonstone.
Nuvie
Produced by: Eric Fry
Website: Nuvie at Sourceforge
Releases:
* Windows Snapshot (all flavours)
* Mac OSX 10.3 Snapshot
* Source

Eric Fry's novel remake of the Ultima 6 engine is also going to be compatible with Savage Empire and Martian Dreams. Though currently being developed under Mac OSX, there are downloadable binaries for both Windows and Mac OSX 10.3 (the developer feels these snapshots might also work under earlier versions of OSX, however this is untested. It is the opinion of Aiera that they will not, simply because of some key API changes that exist between OSX 10.2 and 10.3).

The program source code is also available. Linux users will have to compile Nuvie from source.