Below is a list of previous Google April Fools' Day Hoaxes:
(2007) Google: TiSP (Toilet ISP): Google's new free in-home wireless broadband service.
(2007) Google: About Gmail Paper (New Feature): Gmail, you can request a physical copy of any message with the click of a button, and we'll send it to you in the mail.
(2006) Google Romance: "A place where you can post all types of romantic information and, using our Soulmate Search™, get back search results that could, in theory, include the love of your life. Then we'll send you both on a Contextual DateTM, which we'll pay for while delivering to you relevant ads that we and our advertising partners think will help produce the dating results you're looking for."
(2005) Google Gulp (BETA) with Auto-Drink: A line of "smart drinks" designed to maximize your surfing efficiency by making you more intelligent, and less thirsty.
(2004) Google Copernicus Center: Google is interviewing candidates for engineering positions at our lunar hosting and research center, opening late in the spring of 2007. This unique opportunity is available only to highly-qualified individuals who are willing to relocate for an extended period of time, are in top physical condition and are capable of surviving with limited access to such modern conveniences as soy low-fat lattes, The Sopranos and a steady supply of oxygen.
(2004) GMail was announced on April 1, 2004 and people thought it is another hoax!
(2002) PigeonRank: The heart of Google's search technology is PigeonRank, a system for ranking web pages developed by Google founders Larry Page and Sergey Brin at Stanford University.
(2000) Google MentalPlex: Search technology that read user's mind to determine what the user wanted to search for. No need to actually type in the search query.
Other April Fool's sites: