"Anyway, we've got to go, Lee Jordan reckons he's found a new secret passageway out of the school."
-- George Weasley (PS9)
There are seven known secret passageways that Harry learns about after acquiring the Marauder’s Map. Fred and George Weasley tell Harry that Filch knows about four of them, and they warn him against the other two (the one behind the mirror on the fourth floor and the one underneath the Whomping Willow). The last one, the one behind the one-eyed witch statue, leads into the Honeydukes cellar and is usable.
One of the secret passageways takes knowing students from Hogwarts to Hogsmeade, the nearby village. It can be accessed via the hump of the one-eyed witch statue on the third floor. It leads into the Honeydukes cellar.
Sirius Black remarks to Harry that there once was a pretty roomy secret passageway behind a big mirror on the fourth floor, though Fred and George told Harry that it is blocked.
Another secret passageway leads to the Shrieking Shack and is underneath the Whomping Willow.
The other four passageways on the Marauder's Map are known by Filch, so Harry is advised by Fred and George not to use them.
There is a secret passageway that leads to The Hog's Head, regulated by Aberforth Dumbledore. The passageway is behind a portrait of Ariana, his deceased sister.