ONScripter is a game engine for making visual novels based off of NScripter, another
VN engine that was popular around 10-20 years ago. NScripter was the engine running many
visual novels, both popular and amateur alike. Examples of VNs created with NScripter or
its derivatives include:
ONScripter itself aims to be fully compatible with regular NScripter games, only on
an open-source engine. Regular ONScripter is still maintained, however it doesn't work
well on modern computers and it's only in Japanese. There is a fork that provides
English support and better modern functionality called ONScripter-EN, and it will be
the derivative I'm talking about when I say "ONScripter" unless otherwise specified.
This engine isn't used too much nowadays, since most people use Ren'Py or any other
free engine you can find, but I think it can still be worth using, even if only to have
an 'old-school' feeling to your game.
Have some images I found on the front page of Google that show you a quick rundown of what to expect with ONScripter:
Here's a link to the most recent version of ONScripter-EN and its tools if you want it yourself.
and here's a guide if you want to figure out how to use it.