This is an experimental backport to Minecraft 1.6.4 of Entity Culling Unofficial (which is a port of Entity Culling).
This mod improves performance by using asynchronous calculations to determine obstructed entities and tile entities, in order to prevent them from rendering. What this means in practice is that entities and tile entities that you can't see, will not lower FPS by being rendered regardless.
For more information, check the original mod pages linked at the top.