“Einstein puzzle” by Flowix games, copyright 2004-2005

I play it from time to time, to see if I can still complete it in under 4 minutes and my brain isn’t slowing down too much with age.

Technically, Dwarf fortress is the oldest game I still play, but I only got fully into it thanks to the non-free Steam version.

I use Calibre to manage my librairy on my NAS, and COPS (https://github.com/seblucas/cops) to access it from anywhere. COPS just read the Calibre database saved on the NAS, and displays it as a self-hosted web site with all categories (authors, ratings, languages,…) and download links.