Although I tinkered with level editors back in the 1990's, specifically the Doom and Duke 3D editors, I didn't really pay much attention to what I was creating.
However, once I started to learn level design in ernest, I didn't want to stop. And although my 'for fun' editor time is currently quite limited, I still retain an urge to create... and so I occasionally do.

Unreal Tournament 3: A simple scene because I wanted to play with the UT3 editor.

Gears of War: A cathedral and street and an excuse to play in the GOW editor.

UT2004: Several levels where I learned the editor and became involved in the Unreal community.

Unreal Tournament: A small level because I wanted to toy with an older version of the editor.

Doom: My earliest explorations of a game editor.