Here are two specialized engines:
It is specialized for games that take part in a voxel world (i.e. the world is made of mostly cubes in a grid). Obviously heavily influenced by Minecraft, although not meant to be a clone. Minetest itself is written in C++, but games are coded in Lua.
This is specialized for the creation of action RPGs. Written in C++. The games seem to use a custom system. Although Flare calls it "mods", these can actually be full-fledged games.
(Note: Just because the scope of these engines is limited, that doesn't mean they aren't engines. Domain-specific engines are still engines. In both cases, you can create new games.)