Multiple towns, multiple players: If a friend lends you a memory card that contains their town data, you can plug it into the second slot on your GameCube and then hop on the train to visit their town. Other towns usually have a different fruit and new villagers, plus its very own shop run by Nook with a different selection. And when you visit someone new may move from either town to the other.
The many advantages to town hopping: You can meet the other town's villagers or even meet up with old friends who moved from your town to the one you're visiting. You can even bring some of the fruit from your friend's town to yours to plant and make fruit trees. Nook only pays 100 Bells for the natural fruit of your town, but he'll pay 500 Bells for foreign fruit. Don't be too hasty to sell them though, plant them instead to make fruit trees to get a continuous cash crop. You may even find some things in a friend's town that you wouldn't find in yours. You could even make an inter-village challenge between your town and your friends, like the first one to 15 villagers or the first to fill the entire museum.