Renewable resource

While running out of resources isn't likely to be a factor because of infinite map generation, this is a list of resources that can produced over and over without having to explore for more:
 * Tree/Wood - Trees can be grown just with a Sapling, Dirt and sufficient light.
 * Cloth - Obtained from Sheep, or in a less efficient way by crafting from String obtained from Spiders
 * Cobblestone/Stone - Infinite Cobblestone may be created by directing Water and Lava springs into each other.
 * Reed and Cactus - Can be regrown.
 * Ice and Snow - Ice and snow blocks could be obtained infinitely prior to Halloween Update, by freezing a block of Water and crafting Snowballs respectively.
 * Obsidian - Portals may spawn other portals nearby which can be harvested for Obsidian.