Adding two of my favorites:

XMorph - blends two images together: https://xmorph.sourceforge.net/home/gourlay/public_html/Xmor...

ACM - Aerial Combat Simulator: https://www.gnu.org/software/acm/

xmorph was awesome. i'd love to see a more modern version of that that produces better quality results.

XBattle is a great game and seems ripe for re-discovery and re-implementing, maybe as a mobile game. It shares some elements with RTS games but had its own vibe.

There've been a few XBattle clones in the last 20 years, mostly Flash games, but none really took off. It's a great game mechanic.

xtank as well!

Yes I remember playing that one too.

And xconq

Bringing the grognard experience to computers.