One of the programs I'm the most pleased with was a small screensaver .COM program I wrote for DOS (for personal use). Pressing both shift keys at the same time toggled a blank screen screensaver on/off. There was a similar program released as part of Norton utilities but I got my .COM file smaller than theirs using assembly. After relocating the loader code or was it PSP? Cannot remember, it was something like 150'ish bytes of code in memory, maybe less :D

I wrote a similar TSR (Terminate and Stay Resident) program that would reboot the machine if the letter E was typed. We had a few of us at school always messing with each other haha