I have thoroughly enjoyed Google's set of pranks. They make me eagerly await April 1st each year. Of course, I am a geek.

It started with (and revolved around) CADIE, a sentient AI being that developed a late 1990s style web page and loves pandas and emoticons.
http://www.google.com/intl/en_us/landing/cadie/
http://cadiesingularity.blogspot.com/
Of course, her blog was only meaningfully visited by herself, as a clever write-up from the Google Analytics team disclosed, just before she took over the grid.
http://analytics.blogspot.com/2009/04/cadies-google-analytics-reports.html

Then, Gmail Autopilot was unveiled. It automatically answers your emails for you, and you can adjust how many typos, capital letters, and emoticons, etc., you want it to have so that it seems realistic. The examples were classic: answering the Nigerian spam with the classic response "I will do everything in my power to aid you and accept the $25M. Please find my bank account numbers... mother's maiden name... and social security number," The response lying to your therapist about ceasing to lie cracked me up, too.
http://mail.google.com/mail/help/autopilot/index.html

Youtube lets you flip videos upside down, that way they are played correctly if you're in Australia...
http://www.youtube.com/t/new_viewing_experience
Add &flip=1 to the end of any page to get the nice feature.

Google Docs uses CADIE to type your reports, essays, and spreadsheets--on Demand. You just type the first few words, like, "the theme of Wuthering Heights is" and CADIE will fill in the rest of your paper. She'll even add subliminal messages to your presentations.
http://www.google.com/google-d-s/cadie.html