Mods that change the gameworld itself in some way
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Minimap: Adds a minimap to your screen so you can (finally) figure out where the hell you're going.
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Sign Tags: Makes signposts more user friendly, with different fonts, colours and even variable info.
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Terrain Tweaker SMP Mod: The mod tweaks the settings in the terrain generator allowing for things like mostly flooded worlds, having the Nether blocks mineable in the real world, Muddy swamps etc. Achieved via some simple additions to your server.properties file.
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Minecart Mania!: Makes Pressure Plates act like straight/intersection tracks. Adds booster and brake blocks, allowing easy manipulation of minecarts.