Fysiks Fun is a work in progress that aim to more realism to the minecraft world and to make it come alive with natural phenomena. It does this by adding a number of physics effects as well as by adding a few ecology based effects.
The former category of effects include making all liquids finite and to flow in a "natural" way, a full world computation of forces and blocks that fall/break depending on the forces acting on them, and trees that fall sideways when you chop them down. All of these effects are applied to every block in the world (ie. all the water in oceans, all the blocks under/over the player in the world).
The later category of effects include plant life that consume water and that spreads by themselves, and rewritten AI routines for animals to eat and spread in the world, an eco-system for the water coming from rain, falling over the terrain humidifying underground, evaporating into steam based on heat and sunlight and finally coming back as rain again when the steam has moved with the winds.
The mod aims to be compatible with most other Forge mods.
For further details please see the Minecraft Forum thread
You need forge version 965 (for MC 1.6.4) installed. Drop this repository archive into /forge/mcp/src/minecraft/mbrx where is the root of your forge installation. Ie. the following file should exist: /forge/mcp/src/minecraft/mbrx/ff