Master Materia
If you master all of the Materia in a single group, revisit Bugenhagen's planetarium and examine the Huge Materia that matches the category you've mastered. Doing so will give you the master Materia for that group. Master Materia basically equip you with all-or almost all- of the spells, summons, commands, or effects from that group. For instance, the Master Summon Materia equips you with every summon spell and enables you to use each of them an unlimited number of times, providing that you have the MP to do so.