Some of these are not obscure at all, but absolutely standard. Xterm was the shell, I’m not sure there were even alternatives in early X. Xbiff+Xload+Xclock used to be on everybody’s desktop. XV was the main image viewer on X for a long time.
Anybody remember xvacuum? I guess it’s truly obscure since it didn’t make the list. One of my favorite tricks to play on other people in the lab, before X security was a thing, was to run xvacuum on other people’s displays ... it slowly sucks everything on the screen into the mouse cursor.
If the page maintainer is reading, Xsnow has a calculator in the screenshot covering the snow. This one is a bit obscure, but I loved xsnow, so I think it deserves a calculator-less view! :)
It's really cool image editor based on X11, that still is obscure competitor to GIMP and Krita ;-)
Not only for Linux, but for FreeBSD/OpenBSD and Mac OS too!