Lithium is a general-purpose optimization mod for Minecraft which works to improve a number of systems (game physics, mob AI, block ticking, etc) without changing any behavior. It works on both the client and server, and can be installed on servers without requiring clients to also have the mod. Link to Lithium
Phosphor is a Minecraft mod which works to optimize one of game's most inefficient areas-- the lighting engine. It works on both the client and server, and can be installed on servers without requiring clients to also have the mod. With Phosphor, the amount of time the game takes to generate chunks can be halved for some dimensions, and frame stuttering experienced while traversing the world can be significantly reduced. Link to Phopsher
Both of the mods are open source.
