Playing through FF7 for the first time, there's a lot of special command materia that I don't even know what the hell it does.
Deathblow?? Manipulate?? Morph?? Fuck if I know.
Deathblow does a critical hit but has lower accuracy than a normal attack. Some characters have weapons with 255 accuracy and they can make great use of it.
Manipulate lets you control the enemy and is necessary for obtaining some Enemy Skills, such as White Wind and Big Guard, which are two of the best spells in the game.
Morph changes the enemy into an item. There are some rare items that can only be obtained in substantial quantities via Morph, such as stat-boosting "source" items.