Improved rock/mountain textures
Increases the resolution of the rock/mountain textures from 1024 to 4096 in both directions and adds more detail to them. These textures are used for most exterior rocks in the game from what I've seen.
Comparison screenshots, click for full size:

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Install:
Copy the downloaded textures folder into the Skyrim data folder.

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Uninstall:
Delete the folder "mountains" from the Skyrim data\textures\landscape folder.