from kaisernet.fka.cx:

ONScripter is a Free & Open Source implementation of the NScripter
novel game engine, based on SDL and licensed under the GNU General
Public License v2. It allows many popular novel games to run on a
plethora of platforms such as Linux, BSD, Mac, OS/2, and a number of
game consoles; most importantly, it can be easily modified by the
community.

ONScripter-EN is a fork designed to handle English scripts containing
1-byte characters in a better way. Although the original ONScripter is
still maintained, it lacks fully functional English support,
DirectSound support on Windows, PNG alpha mask detection, window
scaling, and many implementations of effect DLLs -- all of which were
added to ONScripter-EN long ago. ONScripter-EN was originally forked
by Insani, passed on to Haeleth, and ended in the hands of Uncle Mion
of Sonozaki Futago-tachi.

ONScripter-EN will not start without a default.ttf font file in the
game folder. Since the engine displays text only with fixed width, and
some games like Umineko and Tsukihime use Japanese or custom glyphs in
their English translations, Sazanami Gothic is the standard-use font
for most releases.

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You should only have to drop ONScripter.app into the folder of a
visual novel made in the NScripter engine. A simple way to test if
this works, download [Narcissu][1] and drop it in there. It should run
the game properly. There's a few other visual novels that use this
engine that you can play with this, such as Tsukihime and Umineko.

More information should be located [here.][2]

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Download 1 is ONScripter-EN for mac 20110628, the latest version.
Download 2 is ONScripter Insani for mac 20060724 tested and works on
OSX 10.4

Compatibility
Architecture: PPC

  [1]: http://macintoshgarden.org/games/narcissu
  [2]: https://kaisernet.fka.cx/onscripter/