Kill history hud
Giga_M

adds a death history on the screen

tides & floods
Lemente

[WIP] [Warning: read full description] Adds the /sealevel command to rise/lower the sealevel.

Portal gun
AiTechEye

Portals and stuff

Ambient Sounds
SimpleDoodle

Adds environment-specific ambient audio and random atmospheric sound effects

Mud
Blocky Player

Adds stylish mud blocks and even blocks you can walk through. Setup traps, secret rooms and much more!