KOReader’s file browser is text-based by default (efficient, but ugly). CoverImage adds high-resolution book covers to the file browser. More importantly, it has a "Fetch missing metadata" function that scrapes Google Books or OpenLibrary to pull covers for your side-loaded books.

(Offline Article Reader)

It is not a "reading" plugin, but for techies, it is a lifestyle.

Displays granular data like how long a specific chapter will take to finish based on your current reading speed. Progress Widget

(Color Temperature)

Improves text justification, especially on narrow screens, reducing large gaps between words and making paragraphs easier on the eyes.