SlackBuildsOrg/system/ksh-openbsd

This package is a patched version of the OpenBSD ksh, which is based on
the original PD-ksh, but heavily modified and maintained by OpenBSD
developers. "Patched" means ported to Linux.
You can download the source package from a mirror listed in the
slackbuild's info file, or you can always checkout the latest source
from OpenBSD's repository.

PD-ksh is a clone of the AT&T Korn shell.

PD-ksh has most of the ksh88 features, not much of the ksh93 features,
and a number of its own features.  It is free and quite portable - you
should be able to compile it easily on pretty much any unix box.
The vi editing mode is better (IMHO) than that of ksh88 or ksh93
(command/file completion using tab (optional), less buggy).