Sundial Waypoint

To track down a secret site or object, characters must observe a beam of sunshine or moonlight, or a shadow cast by such light, which indicates its location. Often this is only possible at a specific time of day and year, although this limitation may be ignored for plot convenience. That said, the idea that you have to race to get the right Plot Coupon to the right place at the right time (Which will always be really soon after you find it) makes for great drama, especially if you're racing someone else to be there at the right time.

If the right day is specifically required, expect a cloudy sky to clear up at the last possible moment, so the characters won't have to wait a year to watch for it again.

Examples of Sundial Waypoint include:

Film

In Raiders of the Lost Ark, the Staff of Ra must be placed in the Map Room at the right time of day in order for it to throw light upon the site of the Well of Souls.
Titan A.E. uses this around 1/3 through the film.
One clue in National Treasure requires observing the shadow of a particular landmark at the right time of day. The filmmakers naturally forgot that the sun's exact position would also depend on the day of the year.
In the first The Land Before Time movie, this is how Littlefoot's mother knew the way to the Great Valley.

Literature

In The Hobbit, the keyhole of Lonely Mountain's hidden entrance is visible only at the right time on Durin's Day, a dwarven holiday.
In Jules Verne's Journey to the Center of the Earth, the entrance to the passage leading Beneath the Earth is in a crater which is touched by the shadow of a mountain peak at noon during the last few days of June.
Common in riddles in the Redwall series, such as in Mattimeo when the entrance to an underground city is located by following the shadow of a pine tree.
In a novel of Sherlock Holmes, the eponymous character finds the old crown jewels by the shadow of a tree. A minor twist is that he has to adjust for centuries of growth.

Live Action Television

In the miniseries Treasure Island In Outer Space, the location of the treasure is indicated by the shadow of a particular spire of rock at a particular time of day.
The two-parter Cliff Hanger that bridged the first and second seasons of Criminal Minds involved the Serial Killer of the Week leaving elaborate clues for the team. One of these included a riddle that involved Middle English terms for times of day, and the shadow of a sword which is lodged in a corpse. Given that there is only one window in the apartment, they overthink this one and use a flashlight to find the exact spot.
In the CSI New York episode "Manhattanhenge", the search for the Compass Killer's lair and his next target is guided by the suspect's obsession with angles of sunlight over the city. Subverted in that Sheldon calculates in advance where the sunlight will fall, rather than waiting to observe it.
In the Made For TV Movie The Trial of The Incredible Hulk, Matt Murdock has a clerk use a map of the city, a ruler (representing a specific building), and a table lamp to figure out the location of David's flophouse. David had mentioned the Fisk Building blocking the sun.
They tested this on MythBusters and found that it works for a few minutes at most (see the Real Life examples).
The Anasazi calendar (mentioned below) makes an appearance in an episode of Carl Sagan's Cosmos.

Tabletop Games

Used in the Call of Cthulhu (tabletop game) supplement Terror from the Stars, for the adventure "The Temple of the Moon". In the temple there is a light shaft that allows moonlight to enter, and a pool with a map on the bottom. If the Tablet of the Moon is placed atop the light shaft at midnight under the light of the full moon, the moonlight will be refracted into a bright point of light on the map, showing the location of the main temple of Shub-Niggurath.

Toys

Bionicle: subverted in the Mata Nui Online Game; the Great Sundial is, of all things, underground, so Takua has to use a gnomon and a Lightstone to simulate 4 o'clock on it to open it.

Western Animation

In Avatar: The Last Airbender, Aang's first communication with Roku depended on sunlight shining on Roku's statue. And when Aang and Zuko needed to enter a Sun Warrior temple, it had a lock of this kind (sunlight striking a specific place) on the doors.
Used in Scooby Doo Camp Scare, to find a 1920s gangster's hidden loot.

Video Games

In the video game Scratches, the tip of the tower's shadow indicates the location of Mrs. Blackwood's grave, although it doesn't show up clearly enough for you to use this clue until the weather clears up.
- Also, to reveal a clue, you must clear a pathway for a beam of sunlight to fall upon a written message, which can't be made out otherwise.
In Nazi Zombies, a ray of light points to the location of the Mystery Box, although it's not actually from the sun since it's visible in night-time maps.
In an artificial-light variant, to find one exit in Uru: Ages Beyond Myst requires turning off lights until a specific shape of shadow forms on the floor, that indicates the route to the doorway.
Subverted in Kingdom of Loathing, in which you are sent on a (very long) quest to build a staff with a gem which will focus light onto a map of the kingdom showing where "The Holy MacGuffin" is buried. When you do it, it instead acts as a Solar-Powered Magnifying Glass, setting fire to the map, and revealing a trap door underneath.
One of the puzzles in Soul Reaver relies on this. In a twist, it requires you to do a typical mirror puzzle to get the beam where you need, then obscure it - which allows you to walk the shadow across a chasm.
One of the clues in The Elder Scrolls: Oblivion is only seen when the sun shines from a certain angle (at a certain time of day) on a gravestone.
In Sonic Adventure's "Lost World" stage, set in old ruins, Sonic comes across a pitch-dark room except for the entrance, which is bathed in sunlight. He needs to find mirrors and point them towards one another at the right angles; the path the sunlight takes as it reflects from mirror to mirror is the path Sonic needs to follow to get to the other side of the room.
Eternal Darkness uses this twice. First, as a wholesale reference to Raiders of the Lost Ark, Michael Edwards must use a Staff of Ra to guide sunlight over a scale model of a city. Second, when back in Alex's chapter, she must use a similar trick in the Roivas mansion's observatory to open a special lock.

Real Life

The clues in the book Kit Williams Masquerade led to Real Life buried treasure (a gold hare), which could be found at the tip of a shadow of a particular cross at midday of the equinox.
Once rebuilt, the buildings of New York's World Trade Center will be arranged so that, every morning on September the 11th, a commemorative beam of sunlight will fall precisely on the spot where the first 9/11 airliner struck the Twin Towers.
Some ancient pre-Columbian archeological sites have this effect on certain dates: "the Chichen-Itza's equinox shadow serpent" and the "Anasazi Calendar on the equinoxes and solstices" are some examples.