Open Cobalt: open-source virtual world browser and toolkit
web.archive.org
web.archive.org
Flash games that one can't play without a flash player, online games that can no longer be played without company run servers, modern arcade machines that will no longer run without a backend server, etc.
I would really welcome if more people though about how to preserve out current digital culture for the long run...
Just in case you hadn't come across Flashpoint: https://bluemaxima.org/flashpoint/
Since early 2018, Flashpoint has saved more than 100,000 games and 10,000 animations running on 35 different platforms.I saw someone in another thread recently mention that it should be a law that if you shut down your online game, you have to release the source code for the server and client. I don't entirely disagree with this notion.
If you sell people a product and then decide to stop supporting it, why would it hurt you to let your customers support it.
Edit: It seems SecondLife is slightly older than I though (2003, versus Croquet's 2005)