Playing DOOM, which I hid from my hyper-religious parents, was always a blast. It trashed the battery so I kept it plugged in - it would get very hot. Probably terrible for the device but I was a kid and wanted to slay some demons.

There was a Mandelbrot viewer that was pretty cool. Lots of stuff I'm probably forgetting.

It also functioned as a device for adult images that I would dual boot into. Not the best for my young brain most likely. Still, I have plenty of nostalgia around using computers to solve parental problems. Or creating more because I didn't understand partitioning, boot loaders, or really anything when installing Ubuntu on a family machine. :)