Campaign house rules that don't belong to any particular category and are too small to warrant their own.
Racial Hit Dice
Abilities and DCs that key or scale off of racial hit dice uses the sum total of both racial hit dice and hit dice from class levels. If a creature has no racial HD, then only HD from class levels are used.
Size and Land Speed
The default land speed for a Medium creature with two legs is 30 feet. Unless otherwise stated, this speed is decreased by 10 feet every size category smaller, and increased by 10 feet every size category larger. For every additional pair of legs, the creature's land speed increases by 10 feet. Effects that change a creature's size also changes their land speed for their new size.
Speed Less Than 10 Feet
If your speed is already at 10 feet, and circumstances further reduces it, your speed is reduced to 5 feet for the first 10 feet your speed is reduced. For every 10 feet your speed is reduced, your speed remains at 5 feet, but the number of move actions to move to an adjacent square increases by 1.
Stacking Natural Attacks
Abilities, templates, or other features that grant a natural attack that you already possess instead increases your natural attack's damage one size category larger. Multiple sources that grant the same natural attack stack together to determine the number of times to increase the natural attack's damage size category.
Stacking Size Changes
Effects that change your size from different sources stack together.