The Long Range materia in the Mythril cave after the Midgar swamp is often missed out on. When you enter the cave, go right to a seemingly dead end. There is a box up on the mountains; you have to climb there to get to it. But the path is hard to see so turn on the indicators, which are green when showing ladders, and red when showing exits. Follow the path and grab the materia. If equipped to a character, it gives their weapon the same effect in the back row as it does in the front row. Useful for Cloud especially; not so much for Vincent, Yuffie and Barret.

It's by no means essential, but I find it really useful.