Possibly not old enough to be included in that list, but my oldest piece of desktop software I always run on my main machine is GKrellm with BubbleFishyMon as system load monitor.
This is giving me strong nostalgia, I loved gkrellm ~20 years ago because of how densely it showed information. I stopped using it eventually because it used a non-trivial amount of CPU just displaying stuff.
http://gkrellm.srcbox.net/