The Founders Under the School

Element (House)	Quote	Source
Air (Ravenclaw)	He lowered himself through the hole until he was hanging on by his fingertips. ... And Harry let go. Cold, damp air rushed past him as he fell down, down, down... ... The distant music stopped. There was a loud bark from the dog, but Hermione had already jumped. She landed on Harry's other side. "We must be miles under the school," she said.	Book 1, Pages 276-277 Sorcerer's Stone, Chapter 16, "Through the Trapdoor"
Water (Slytherin)	He lowered himself slowly into the pipe, then let go. It was like rushing down an endless, slimy, dark slide. He could see more pipes branching off in all directions, but none as large as theirs, which twisted and turned, sloping steeply downward, and he knew that he was falling deeper below the school than even the dungeons. ... "We must be miles under the school," said Harry, his voice echoing in the black tunnel. "Under the lake, probably," said Ron, squinting around at the dark, slimy walls.	Book 2, Page 301 Chamber of Secrets, Chapter 16, "The Chamber of Secrets"
Earth (Hufflepuff)	Harry traced the secret passage to Honeydukes with his finger. ... ...the statue's hump opened wide enough to admit a fairly thin person. Harry ... hoisted himself into the hole headfirst and pushed himself forward. He slid a considerable way down what felt like a stone slide, then landed on cold, damp earth. ... The passage twisted and turned, more like the burrow of a giant rabbit than anything else.	Book 3, Pages 194-195 Prisoner of Azkaban, Chapter 10, "The Marauder's Map"
Fire (Gryffindor)	tbd	tbd - Book 7?

Humble Prediction - We have still to see a secret room in Hogwarts related to Gryffindor/Fire.

UPDATE - 7/19/07:
Here are two guesses on the location of the secret Gryffindor room:

Guess #1) Through the fireplace in the North Tower (Trelawney's classrom). This ties in with Gryffindors, Towers and fire as well as the mention of the fireplace in her classroom.

Guess #2) Through the Defense Against the Dark Arts classroom - thus explaining another reason Voldemort wanted the Defense Against the Dark Arts job.