Junon
Location

The military base / town of Junon plays a pivotal role in the game and during a standard playthrough most players will end up visiting it a number of times. Junon is located on the western coast of the eastern continent and it is the last town that you visit before crossing the ocean over to the western continent in Disc 1.

Junon on the World Map
Junon cinematic

The first visit to Junon is after the team emerges from the Mythril Mine and after the visit to the Chocobo Farm. The team leaves Junon by way of the Cargo Ship which then takes them to Costa del Sol.

The second time you arrive at Junon is after the conclusion of the events at the Whirlwind Maze. Tifa, Barret and the rest of the team arrive at Junon without Cloud in their company.

The third and final visit, as far as the main storyline goes at least, is when the team passes through Junon on the way to the Underwater Reactor. This is when they are in pursuit of the third piece of Huge Materia and just before the Submarine Fight in the Shinra Sub.

There are a number of items that you can pick up in the various houses and apartments in Junon including:

Shiva Materia
1/35 Soldiers
Mind Source
Luck Source
Power Source
Guard Source
Speed Source
Enemy Skill Materia

There are also a ton of shops in the area. The shops have additional items once you have concluded your second visit at Junon once Meteor has been summoned by Sephiroth:

50 gil
300 gil
80 gil
50 gil
100 gil
100 gil
500 gil
2,000 gil
3,200 gil
7,000 gil
6,200 gil
3,500 gil
6,800 gil
2,000 gil
3,500 gil
1,800 gil
3,200 gil
2,800 gil
3,200 gil
2,200 gil
6,000 gil
3,000 gil
3,000 gil
3,200 gil
3,000 gil
1,800 gil
4,200 gil
3,300 gil
6,000 gil
2,300 gil
2,800 gil
3,300 gil
1,400 gil
1,200 gil
1,300 gil
1,300 gil
2,800 gil
500 gil
1,200 gil
7,500 gil
1,000 gil

* Note that there are multiple Weapon Shops and Materia Shops scattered throughout Junon.

The map below shows the location for Junon on the World Map:

Junon Location