Calculator

The Calculator accepts many Humorous units of measurement, including the beard-second (5 nm), Potrzebie (2.2633mm), ngogn (11.5938151ml), blintz (36.4253863g), donkeypower (0.3353 hp), etc.
Searching for "the loneliest number" will make the Calculator answer 1, a reference to the song by Three Dog Night.
Searching for "the answer to life, the universe, and everything" will make the Calculator answer 42, a reference to Douglas Adams's novel The Hitchhiker's Guide to the Galaxy. In order for this Easter egg to be successful the phrase must be entered in lowercase and without the quotation marks. The same reference is made when speaking to an artificial intelligence named Meliza next to "the face on Mars" in Google Earth. When the number 42 is typed, the AI produces the output "42 is the answer to the ultimate question about life, the universe, and everything."
Searching for "the number of horns on a unicorn" produces the answer "1" in the Calculator.
(Google actually reads the strings "number of horns on a unicorn" and "answer to life the universe and everything" as numbers. One can also use these phrases in the Calculator. For example searching for "number of horns on a unicorn usd in euro" produces the current dollar/euro conversion for one dollar, or searching for "answer to life the universe and everything*5" produces 210.)

Searching for "once in a blue moon" shows the result "1.16699016 × 10-8 hertz".

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Various Google services also hide Easter eggs meant to be amusing entertainment.

Searching for "recursion" shows "Did you mean: recursion".
Searching for "anagram" shows "Did you mean: nag a ram".
Searching "ascii art" will turn the Google logo into ASCII art, but only in lowercase, even though uppercase is technically correct.
Before and during the span of the 2010 FIFA World Cup, searching for various terms which Google has statistics related to, such as "world cup", "world cup usa vs england", or "world cup brazil", causes the result index at the bottom of the page to read as "Gooooooooooal!" instead of "Goooooooooogle".

Google Maps/Earth
When asked how to get from a location in North America to a location in Europe or Africa, Google Maps included the instruction "Swim across the Atlantic Ocean".This Easter egg is now removed.
When asked for directions from North America to Australia or an island in the Pacific ocean, Google Maps includes the instruction "Kayak across the Pacific Ocean". This also worked in Google Earth.
When asked for directions from Japan to China, Google Maps includes the instruction "Jet ski across the Pacific Ocean".
The measurement tool in Google Earth allows users to measure distance in smoots, a unit of length derived from a tradition at MIT. Smoots are also recognized by Google calculator: 1 smoot in m gives the result 1 smoot = 1.7018 meters.
On Google Earth, tapping out ctrl-alt-A will activate a hidden flight simulator. (This is now an official feature.)
Going on Google Street View, and heading to the rear of the company's Googleplex headquarters in Mountain View, California, the Google Street View's production team can be seen.
Google Earth's search example is the Google inc. headquarters. (37 25' 19.1"N, 122 05' 06"W)
Dragging the Google Street View "Peg man" onto Telegraph Avenue in Berkeley, CA used to display him with a tie-died shirt. He is also given special clothing for some widely observed special occasions: for halloween he rides a broomstick; for valentine's day he stands upon a heart; and during Christmas week, he becomes a snowman.
The Mars Feature of Google Earth allows you to speak to a primitive ELIZA clone on the planet, by searching for "Meliza".

Google.com homepage
In December 2009, Google added an easter egg on their homepage whereby clicking the "I'm Feeling Lucky" box with no text entered brings up a clock counting down the seconds until 2010. When it reached zero it displayed "Happy New Year" in fireworks.
In December 2009 when typing in a search term including the word "Christmas" the line separating the sponsored links from the normal search is replaced with Christmas lights. Also, when typing in a search term including the word "Hanukkah" the line separating the sponsored links is replaced with a Dreidel design.
From May 21–22, 2010, Google replaced its usual homepage with a playable Google-style Pac-man game to celebrate its 30th anniversary. By pressing Insert Coin (which replaced the "I'm feeling lucky" button) multiple times, you could also play a 2-player Ms. Pacman version. The game is still playable
Google Mail

On the Suggest a feature page for Gmail, it shows "Have Gmail do the laundry" as a suggestion
Above the list of emails in GMail, there is normally an ad, except in the spam folder, where there is instead a link to a recipe containing SPAM™.

Other Eggs
Pressing control-shift-y in Picasa will cause a teddybear to appear on the screen.
Pressing the keys "up, up, down, down, left, right, left, right, B, A" in Google's reader will cause the screen to turn blue and a ninja to appear on the left.
Google offers services in many languages, including several uncommon ones like Swedish Chef's Bork bork bork, Pig Latin, Hacker (usually known as 1337sp34k), Elmer Fudd, Klingon and Pirate
Taking the term Easter egg literally (and perhaps to celebrate the Easter holiday), Google has an official Easter Eggs page.
If one sets the iGoogle theme to the "Beach" option, then at 3:14 AM every morning, the Loch Ness Monster surfaces for 1 minute, then at 3:15 dives back under. The reason for the timing of 3:14 is rumoured to be a tribute to the number pi. Additional 3:14 eggs include the "Seasonal Scape" showing off the Northern Lights, the "City Scape" with UFOs, the "Spring Scape" with a monster, the "Sweet Dreams" with the stars aligning to the shape of the symbol Pi, and the "Tea House" that has spirits in the mist.
In Chrome, Google's Web browser, entering "about:internets" into the address bar brings up a copy of the Windows Screen Saver 3D Pipes, with the title "Don't Clog the Tubes". In the event that this does not occur due to the fact that the screensaver file does not exist in the system, the browser will instead display a gray screen with the title "The Tubes are Clogged!" This has been removed as of the 2.0.169.1 release.
The Google Gear's Firefox add-on description line reads "These are the gears that power the tubes! :-)"
The above two easter eggs allude to former Senator Ted Stevens' "series of tubes" speech on the Senate floor.
During the course of the 2010 FIFA World Cup, Youtube had a button shaped like a football in certain videos that plays a recurring vuvuzela sound throughout the video.
In Google's iPhone and iPod touch search application, swiping downwards (past About) repeatedly in the Settings interface brings up a hidden menu item, called Bells and Whistles, allowing customization of colors, sounds and more within the app. This is now standard on the newest update.
In Google translate, giving "James Bond" and translating it to Chinese will give the answer "007".