Google Doodles, also known as Holiday logos have been used by Google since 1998. The first was for the Burning Man festival. When clicked, the doodles lead to a specific search query that is relevant to the doodle.
Logos are produced for birthdays of famous artists and scientists, historical anniversaries, national and international days, and contests for designing a Google Doodle.
Google has also built interactive doodles. In May 2010, to celebrate the 30th anniversary of Pac-Man, Google added a playable version of Pac-Man to their home page. The game can still be played at google.com/pacman. (view screenshot). A more recent example in June 2011 was the doodle for Les Paul's 96th birthday, featuring a virtual guitar you could string. Samples can be recorded and shared with friends. Another recent one was the Google Snake game, in celebration of the Chinese Year of the Snake. It also can still be played at http://elgoog.im/snake/.
5 Most Recent Doodles
The full list of logos are available at google.com/doodles.