GeoStrata by Reika
GeoStrata is a small mod to diversify underground world generation, making mining and caves more interesting, while simultaneously adding many aesthetic and functional building materials.
User config-based controls for geographical features, including land sizes, ocean amounts, climate sizes and incidences, biome sizes and incidences, and mountain chains
Geologica by noogenesis
Adds realistic geology in the form of new types of rocks and ores. The distributions are completely customizable, either via the GUI during world creation, or by directly modifying the CustomOreGen config.