Apart from UTF-8, the piece of software he designed I used most was the sam text editor. An opinionated and rather quirky design, with some internals that made quite enthralling reading for me at the time: http://doc.cat-v.org/plan_9/4th_edition/papers/sam/

It's an interesting editor, especially the split between the editor itself and the graphical frontend. That split has helped me work on some very limited systems, thanks to `sam -r`.

Personally, I prefer acme; it's my daily driver editor.