Original Lua version of the Valleys mapgen, before it was merged in Minetest's core in January 2016. It provides some more features than the core version, like new plants, new dirt variations and a soft biome system.
Valleys Mapgen
A map generator with valleys, rivers, mountains and many plants.
Custom mapgen Mapgen / Biomes / Decoration Plants and Farming
Great but needs update
This mod makes world nicer and much more realistic than default mapgens (v5, v6, v7...). But, has some problems and issues, needs update for good working on higher versions (in 5.12 does sometines some problems before world starting).