"Which person, which abysmally foolish person wrote down this week's passwords and left them lying around?"
     - Minerva McGonagall

All of these are passwords to get into Gryffindor common room unless noted otherwise.

balderdash (GF12, GF14)
banana fritters (GF25)
caput draconis (SS7)
cockroach cluster (Dumbledore's office) (GF29)
fairy lights (GF22)
flibbertigibbet (PA15)
fortuna major (PA5)
lemon drop (Dumbledore's office) (CS11)
oddsbodkins (Sir Cadogan thought this one up) (PA12)
pig snout (SS9)
pine fresh (GF ) (Prefect's bathroom on the fifth floor)
pure-blood (Slytherin common room) (CS12)
scurvy cur (Sir Cadogan thought this one up too) (PA11)
sherbet lemon (Dumbledore's office) (GF29)
wattlebird (CS5)

open up - all it takes to open the Chamber of Secrets, spoken to one particular copper tap in Moaning Myrtle's bathroom. Of course, you have to say the words in Parseltongue... (CS16)