Z-machine versions

There are at least 10 different versions of the Z machine out there:

Versions 1-6 were created by Infocom in the 1980s.
Graham Nelson made some minor changes in the 1990s to allow for larger story files, creating versions 7 and 8 of the spec. Most, if not all, modern Z machine interpreters support z7 and z8 story files.
In the early 2000s, there was a great deal of discussion about new version 9 and 10 extensions, but these were never resolved.
Adventures which are too big for even z8 can use Andrew Plotkin's Glulx virtual machine, which is generated using the same toolchain used to make most Z-machine adventures. Glulx has been around since the early oughts (first 2000s decade).
In 2013, Zzo38 created (another) version 9 and 10 of the spec, with a number of extensions allowing even larger story files, finally giving Z-machines XOR support, and many other changes. These versions are not described in the 1997 Z-machine specification 1.0, nor in the 2006 Z-machine specification v1.1 (the most recent version), nor in the draft Z-machine specification v1.2; the only known Z machine interpreters that support these versions are Zzo38's own Aimfiz and Fweep.

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