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 - $18.50
Lightly Played - $17.50
Moderately Played - $13.75
Heavily Played - $11.00
Damaged - $7.50
Near Mint Non-English - $18.50
Lightly Played Non-English - $17.50
Moderately Played Non-English - $13.75
Heavily Played Non-English - $11.00
Damaged Non-English - $7.50