The major point of these is that they aren't generic, and they can all be detected through player-DM dialog rather than that boring old d6 roll we used to make every time the party said they were searching for secret doors on some nondescript section of wall.
The shadow knows: A statue with outstretched arms stands in the center of the room. On the wall behind it, to left and right, are lamps in sconces. If both are lit, and all other lights extinguished, the shadows of the statue's fingers converge on a brick or stone in the opposite wall. Pressing this opens a secret door.
Gulp!: A huge monster head, possibly a dragon or demon, is mounted on (or carved from) the wall. Hidden in its gullet is a passage to another part of the dungeon. Depending on its size, human-sized characters may have to crawl. May also involve a slide, because slides are fun.
Seeping portal: Behind this secret door is a room filled with water. Water seeps through the seams between door and wall, darkening the door with dampness so it stands out clearly from the rest of the wall. The door opens into the flooded chamber, so it cannot be opened until the pressure on both sides is equalized. Somewhere in the dry room is a lever or other switch, which will either drain the flooded room or flood the dry room. (Come on, you know you'd rather flood the dry room!)
Hall of mirrors: Several full-length mirrors face each other across a corridor or narrow room, creating the illusion of infinite passages beyond each one. One mirror is simply a glass door with an actual corridor behind it (complete with repeating mirror frames.) It is recognizable by the fact that it and the real mirror opposite it don't contain infinite images of the person looking at them.
Decoy door: A very prominent false door with ornate metal hinges stands next to the well-concealed secret door. The false door swings out from the wall, but there is only solid wall behind it. The secret door swings on the same hinges from the opposite side.
Sinister wheel: A large circular plate is visible in the wall, about as wide as a human's shoulders, with two sets of four small holes on opposite sides - perfect for inserting the fingers of both hands. Blood stains may be visible around the holes. If fingers are inserted, the holes provide a sound grip and the circular panel may be turned. Turning it counterclockwise opens the secret door. Turning clockwise turns the fingers against very sharp blades hidden within the wall, causing 1d3 points of damage and severing at least the little finger of the right hand and index finger of the left.
Ice curtain: Secret passages hidden behind a waterfall are almost cliche, but what about behind a frozen waterfall? Characters peering through the ice with a bright enough light source can see the dark doorway behind it. Getting it “open” requires a lot of noisy smashing or a fire spell. Will re-freeze in a day or so.
Through the flames: In a wall of searing flames (either magical or burning flammable gas welling up from deep in the ground) one section is illusionary. Passing through the real flames causes severe burns (at least 3d6 points of fire damage, or more at DM's choice.) The false flames may be passed through without harm. Careful examination will detect the cooler spot amid the intense heat from the real flames. This would be especially appropriate for a shrine, where the high priest might have awed his followers by emerging from the flames.
Celestial gate: On a pedestal near the wall is a clockwork model of the heavens. The pedestal bears an inscription in an ancient language stating "In the dark of the moon the way shall open." Aligning the bodies in the model so that the moon is in its new phase (relative to the home world) unlocks the door.
Pool portal: The room contains a large, deep pool of water, and the door is hidden in one of the walls of the pool, below the surface. The door opens inward, so the pool must be drained in order to open it. Perhaps there's a plug somewhere on the bottom? This could be the other side of a seeping portal.