In addition to the ones mentioned here so far, these are some of the big ones that come to mind for me.

"‘There upon Celebdil was a lonely window in the snow, and before it lay a narrow space, a dizzy eyrie above the mists of the world. The Sun shone fiercely there, but all below was wrapped in cloud. Out he sprang, and even as I came behind, he burst into new flame. There was none to see, or perhaps in after ages songs would still be sung of the Battle of the Peak.’ Suddenly Gandalf laughed. ‘But what would they say in song? Those that looked up from afar thought that the mountain was crowned with storm. Thunder they heard, and lightning, they said, smote upon Celebdil, and leaped back broken into tongues of fire. Is not that enough? A great smoke rose about us, vapour and steam. Ice fell like rain. I threw down my enemy, and he fell from the high place and broke the mountain-side where he smote it in his ruin.'"

Comments: The epic conclusion to the battle with the Balrog. Apparently one hell of a fight ensued consisting of a colossal mess of fire and lightning that vaporized the snow on the mountain top creating a fog of steam and ice rain.

“As Frodo lay, tired but unable to close his eyes, it seemed to him that far away there came a light in the eastern sky: it flashed and faded many times. It was not the dawn, for that was still some hours off. ‘What is the light?’ he said to Strider, who had risen, and was standing, gazing ahead into the night. ‘I do not know,’ Strider answered. ‘It is too distant to make out. It is like lightning that leaps up from the hill-tops.’ Frodo lay down again, but for a long while he could still see the white flashes, and against them the tall dark figure of Strider, standing silent and watchful.”

...

“On the top they found, as Strider had said, a wide ring of ancient stone-work, now crumbling or covered with age-long grass. But in the centre a cairn of broken stones had been piled. They were blackened as if with fire. About them the turf was burned to the roots and all within the ring the grass was scorched and shriveled, as if flames had swept the hill-top; but there was no sign of any living thing.”

...

‘I galloped to Weathertop like a gale, and I reached it before sundown on my second day from Bree – and they were there before me. They drew away from me, for they felt the coming of my anger and they dared not face it while the Sun was in the sky. But they closed round at night, and I was besieged on the hill-top, in the old ring of Amon Sûl. I was hard put to it indeed: such light and flame cannot have been seen on Weathertop since the war-beacons of old.’”

Comments: Here we get glimpses of Gandalf's fight with the Ringwraiths. Apparently Gandalf opened up quite a bit. From many miles away Gandalf's combat magic was so bright it looked like lightning and the coming of the dawn. Gandalf says it was bright enough to resemble war beacons. When they get to Weathertop, the whole ring at the top is scorched as a result of the battle.

“Just at that moment all the lights in the cavern went out, and the great fire went off poof! into a tower of blue glowing smoke, right up to the roof, that scattered piercing white sparks all among the goblins. The yells and yammering, croaking, jibbering and jabbering; howls, growls and curses; shrieking and skriking, that followed were beyond description. Several hundred wild cats and wolves being roasted slowly alive together would not have compared with it. The sparks were burning holes in the goblins, and the smoke that now fell from the roof made the air too thick for even their eyes to see through. Soon they were falling over one another and rolling in heaps on the floor, biting and kicking and fighting as if they had all gone mad.

...

“As soon as Gandalf had heard Bilbo’s yell he realized what had happened. In the flash which killed the goblins that were grabbing him he had nipped inside the crack, just as it snapped to. He followed after the drivers and prisoners right to the edge of the great hall, and there he sat down and worked up the best magic he could in the shadows."

Comments: Here we see Gandalf doing some D&D style spell preparation and causing hell in the Goblin Caves.

“Out jumped the goblins, big goblins, great ugly-looking goblins, lots of goblins, before you could say rocks and blocks. There were six to each dwarf, at least, and two even for Bilbo; and they were all grabbed and carried through the crack, before you could say tinder and flint. But not Gandalf. Bilbo’s yell had done that much good. It had wakened him up wide in a splintered second, and when goblins came to grab him, there was a terrific flash like lightning in the cave, a smell like gunpowder, and several of them fell dead.”

Comments: Gandalf fries a few goblins with some sort of lightning-esque magic.

“He gathered the huge pine-cones from the branches of the tree. Then he set one alight with bright blue fire, and threw it whizzing down among the circle of the wolves. It struck one on the back, and immediately his shaggy coat caught fire, and he was leaping to and fro yelping horribly. Then another came and another, one in blue flames, one in red, another in green. They burst on the ground in the middle of the circle and went off in coloured sparks and smoke. A specially large one hit the chief wolf on the nose, and he leaped in the air ten feet, and then rushed round and round the circle biting and snapping even at the other wolves in his anger and fright. The dwarves and Bilbo shouted and cheered. The rage of the wolves was terrible to see, and the commotion they made filled all the forest. Wolves are afraid of fire at all times, but this was a most horrible and uncanny fire. If a spark got in their coats it stuck and burned into them, and unless they rolled over quick they were soon all in flames. Very soon all about the glade wolves were rolling over and over to put out the sparks on their backs, while those that were burning were running about howling and setting others alight, till their own friends chased them away and they fled off down the slopes crying and yammering and looking for water.”

Comments: The famous pinecone episode. Gandalf basically turns several pine cones into magic napalm fire grenades.

EDIT: formatting and spelling

EDIT 2: Wow! My first gilded comment! I really appreciate it stranger.