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
Mirror: https://plckthn.me/pebble/barely/pebble-barely.pbw
Mirror: https://plckthn.me/pebble/barely/pebble-barely.pbw
Search for it in the Pebble Store! ;)
Edit: This thread now only supports the Barely watchface, not the V2.
Based on the work by Simon K.