Shrink.

Shrink is a small mod for Minecraft that allows players to change their player's size.

Shrinking to a smaller size allows the player to access places that they could normally not access without breaking blocks.

This mod adds a single item called a Personal Shrinking Device, Shift-right clicking this item will cause your player to shrink down to about the size of a quarter of a block.

The Personal Shrinking Device requires power for any RF/Forge Energy source, This can be disabled in the config file