I was a Macintosh guy for a long time.
I started with the Commodore 64, then a weird Sony word processor, and ended up with a Macintosh LC at the beginning of high school.
I got tired of just games after a while and set about learning other stuff to do with my Mac and that road lead me to ResEdit.
ResEdit was a utility which let you view and modify the resource fork of an application. While the resource fork primarily contained all of the image/icon/text resources for an application, it also contained the executable code for Classic 68k applications.
I figured out how make simple changes on my own (mainly changing to text of menu items to bad words) but I was pretty limited in what I could do.
And then I discovered Zen and the Art of Resource Editing.
They taught magical tricks like changing the Macintosh startup sound (I changed my to the THX sound) or even changing the startup screen.
My startup screen looked like this for the rest of high school.
I got my second Mac in college (where I met my future wife) which necessitated a startup screen update.