A basic calculator made with c++ was my peak before I got distracted with other things.

When I was in uni, i had to go abroad, but i had a project to do (a virtual memory manager). I left my home PC turned on, and then connected with ssh, and did the whole thing with vim, gcc, gdb and valgrind. It was fun and educational

One of my smallest, but a Firefox extension that allows me to adjust playback speed with a slider. Allows for direct and dynamic adjustments. I often watch videos at 1.3x speed.