PADD is the first "proper" software application I ever wrote. I was 16 at the time and wanted to create a private diary on my computer, the wonderful Atari ST. Being a huge Star Trek nerd, I called it PADD (Personal Access Display Device).
It looked like this:

It's not quite on par with the Starfleet version.
Despite its simple looks, there was a fair amount of functionality. It had a working diary, a notebook, an address book, and a lightweight tracker for finances. The application itself also had a login screen to prevent prying eyes, and all entries were stored on disk using a very simple encryption function.
I took some more screens which show:
- The main entry screen
- What an encrypted entry looks like
- The financial screen
Unfortunately I don't have the source code available anymore. My talent for designing interfaces is about the same, but I'm pretty sure my coding ability has improved.