After Cloud recovers from his Mako Poisoning,
go to the basement of Shinra Mansion. Enter the Library and he'll have a flashback about how he became a mercenary.
More Tetra Elementals
Morph a Cactuar (cactus-like monster).
They are on an island in the South, under the West Continent. You need the Highwind or a Gold Chocobo. Get attacked in the sand.
Knights of the Round
Check the Materia Section.
Keep entering and exiting The Gold Saucer Welcome sign
until you see a man between the save point and the house in the background. The most you can buy is 10,000 gil for 100 GP.
If you happen to get 7777 HP exactly on a character,
enter a battle and that character will do 7777 damage with every hit. After the battle, though, that character will only have 1 HP.
Aeris can only be revived using the Gameshark Code 8009CBDEFF03.
She won't talk almost all the time. And it can make the game get glitchy some of the time. There is a real way to revive her, or there was going to be, but it never got totally put into the game.
Wanna be Immortal?
Attach the Final Attack materia to the Pheonix materia.
Not exactly immortality but anytime that character dies, he revives whoever else is already dead. As long as you have enough MP.
Shinra Mansion Safe Combo
Right 36, Left 10, Right 59, Right 97
In the crashed Gelnika plane,
any monster you fight can be morphed into a source. Except, of course, the Turks.
If you get the Sector 5 key to Midgar on the 2nd or 3rd disc,
and you go to the church, you can see Aeris's Ghost for a sec.