Underground Ocean by Reoseah
Small mod that adds large caverns filled with water to world generation.
Underground Vegetation
Miner
UnderlineFix by Axer
Fix the bug which has caused unable to type underline on OSX.
Underp Hangables by masa
Fixes the derpyness of the dropped items from Paintings and Item Frames when they are broken.
Undertale
Sorry
Underwater Chest Bubbles by Squirtle8459
Upon opening chests underwater.. creates bubbles..
Underwater Rails by Hennamann
Made for Modjam 5 adds 2 new rail types and tools intended for use underwater.
Underwater Utilities by Shadowfacts
Helpful utilities for working underwater.
Undestructible by The_Icy_One
Make your structures undestructible.
Unfathomed Depths Mod by Josh029
Ancient Technology of Mysterious Depth.
Ungrab Mouse Mod by Jaideep Singh Heer
This is a simple mod to allow players to ungrab the mouse to do other things on their PCs while the game runs.
Ungui by Mrkol
Declaring a war on GUI screens! This was made as part of MODJAM3. If you are reading this in 2014, then I probably got into top 10. Otherwise I would totally be all sad and stop modding for a couple more months :(
Unhealthy Dying by Mrbysco
Losing health from dying.
UniDict by WanionCane
a mod about unifying all the things.
UniQ by domi1819
OreDict unifier