Speech Recognition SW, first installed somewhere around the time of Sputnik. I don't know the current version but it apparently is updated daily. What I like most about it, is the fact that it runs on virtually no memory; exactly how little is difficult to tell, but I can report that available memory is obviously shrinking with each passing moment, yet it still runs. A little buggy, but does the job. Running on an ancient chemical-electric processor, which is networked by a cumbersome and slow interface (AMP, or analogue-mechanical protocol) with a PowerMac G4.
On the G4, it must be QuickDex, a freeform database dating back to System 7.1. It's the app I got for free from my first Mac Bible CD, somewhere back before there was such a thing as a PPC Mac. Essentially similar to PhonePad, I would guess. I hacked it with ResEdit about 7 years ago so that the text entry window is resizeable to 800 pixels vertical.
I've mentioned this before, but I did run System 6's Font-DA Mover accidentally once. It launched in Classic and appeared to run perfectly. However this is of limited value (and doesn't really apply to the topic), as it has no earthly use after the introduction of System 7. Didn't crash, though.
As for games, I still play Civilisation II on the G4. It runs even better than it ever did on the first computer I used it on, a 1st generation PowerMac.