ocCraft
is a little videogame originated trough a very little OpenComputers game jam. It is currently not much more as the engine but should be easy to modify as long as you have a little practice with lua.
Modding
You can install/create your own texture packs (also with different resolution) as well as mods with new blocks, entities and biomes (world gen)*.
*(technically you can change nearly anything at runtime because the most data are stored in a local table named global which will given to any script at loading.)
GitHub: https://github.com/MisterNoNameLP/ocCraft