Each creature gets +1/+1 for each other creature on the battlefield that shares at least one creature type with it. (For example, if two Goblin Warriors and a Goblin Shaman are on the battlefield, each gets +2/+2.)
"Hup, two, three, four,
Dunno how to count no more."
—Mogg march
Near Mint - $28.200
Lightly Played - $22.600
Moderately Played - $19.800
Heavily Played - $17.000
Damaged - $14.200
Near Mint Spanish - $28.200
Lightly Played Spanish - $22.600
Moderately Played Spanish - $19.800
Heavily Played Spanish - $17.000
Damaged Spanish - $14.200