Barely
It displays very simplified digits, filling the whole screen, with just horizontal and vertical straight lines.
It is invertable too!
Open Source: https://github.com/philplckthun/pebble-barely
Barely V2
Four squares for the time, four for the date, and four for the year, filling the whole screen. Also invertable!
Open Source: https://github.com/philplckthun/pebble-barely-v2
Search for them in the Pebble Store! ;)
Based on the work by Simon K.