The ballpoint pen. Imagine if our choices for writing instruments were still between the permanent, but cumbersome fountain pen and the easy to use but also easy to erase pencil. As someone who handwrites a lot of notes, I can't stress how important it is for me to have something that can write permanently, while still being easy to use, and inexpensive.
I mean, sure a lot of things can be written down as digital notes these days, but imagine of you went to school 10 years ago and all you had to write notes with was the fountain pen. What if you run out of ink in the middle of class, how would you finish your notes? Or what if you were using a pencil while writing an exam essay. You'd have to keep sharpening it every now and then, disrupting your train of thought.
The ballpoint pen just made my school life easier. However, the fact that I always see ballpoint pens lying around everywhere, forgotten and disposed of by its owner just shows it doesn't get a lot of love.