In 1998, while Google was still relatively new on the scene, they had some fun during the Burning Man festival. Fearing that their surveys might crash, they gave a temporary graphical facelift to their logo to notify users of their absence. This Burning Man rendition was the first of many "Google Doodles" to come in the years ahead.
On holidays, I always find a few minutes to hop onto the my computer just to see what whimsical logo interpretation is occupying Google's brand space. It puts a smile on my face every time.
When Google chooses to pay tribute to a certain historical figure's birthday, there's a delight at being surprised and enlightened at the same time. On these days, clicking on the logo will display search results pertaining to the subject of the logo, encouraging the user to learn more about the subject. This encourages activeparticipationwith both the brand and product.
On April 27th, just such a day occurred: Samuel Morse's birthday. It shows the power of the Google brand that it can play with its logo so far as to be unrecognizable, yet still be effective.
A comprehensive Google Doodle collection can be found at Google's archive.