What are some old freeware PC games you've kept playing over the years?

"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.