Google is conducting a contest for designing doodles (special logos).
The theme is "What if ..?".
The rules say "No purchase necessary".
It seems to be for students in USA.

contest:
http://www.google.com/doodle4google

rules:
http://www.google.com/doodle4google/rules.html
  
faq:
http://www.google.com/doodle4google/faq.html

It has been launched one month ago ;-)
http://googleblog.blogspot.com/2008/02/what-if-you-could-have-your-doodle-on.html