Middle School Notes was the first website I ever built in 7th grade, so I wouldn't have to carry my textbooks as I walked to and from school.
I took pretty good notes in class, so I decided to put them up on a website so others could benefit from them, and this eventually ended up being used by everyone in my middle school.
This is also the project that kickstarted my journey with programming, and more broadly sparked the desire to build incredible products that can be massively useful to others.
Here's an archived version of the site: