Here are the locations of the Great Fairy Founatains in the Ocarina of Time and what you recieve from each one of them.
1st Great Fairy Fountain
This Great Fairy Fountain is located at Hyrule Castle. You need the bomb-bag to get it. Walk along the path in the Hyrule Castle field. When you see a indentation in the wall and a rock in the middle of it, walk up and bomb it. Then crawl into the hole that is exposed behind the rock. You will find a Great Fairy after playing Zelda's Lullaby on the triforce symbol. She will give you Din's Fire.
2nd Great Fairy Fountain
To get to this Great Fairy Fountain, go to the peak of Death Mountain. You will need the bomb-bag and the Hylian Shield to make it to the Great Fairy Fountain. Once at the top, look for the different looking wall next to the owls pearch. Bomb it and walk inside. You will receive a magic meter from this Great Fairy.
3rd Great Fairy Fountain
To get to this Great Fairy Fountain, go to the area called Zora Fountain. Behind where Jabu-Jabu was, there is some land with boulders on it. Go over to it. It may not look like it will do anything, but bomb one of the rocks and the entrance will be revealed. This Great Fairy will give you Farore's Wind.
4th Great Fairy Fountain
This Great Fairy Fountain is in the Death Mountain Crater. You will need the Megaton Hammer to reach it. Walk inside of Death Mountain Crater from the entrance from Goron City. You will see a bunch of rocks in front of a tunnel. Smash those boulders up with your Megaton Hammer and enter the tunnel. You will recieve a huge extension in your Magic Meter.
5th Great Fairy Fountain
This Fairy Fountain is my favorite in the game. It is located at Desert Colossus. Once you enter it form the Haunted Wasteland. Walk to the right where there is a big stone wall. There is a small crack in the wall. Bomb it and walk inside the newly exposed entrance. You will receive a very awesome power, Nayru's Love.
6th Great Fairy Fountain
This Great Fairy Fountain you will uncover very late in the game. You must acquire the Golden Gauntlets from the Shadow Portion of Ganadorf's Castle. Then take a left after exiting Ganadorf's Castle and look at the giant black rock. Lift it up and walk inside the cave. You will receive crests around your hearts from this fairy. It will reduce the damage you recieve by one half.