Google Zeitgeist This Month
We update Zeitgeist every month to reflect the changes in each domain. Zeitgeist displays either the top gaining queries in a domain (that is, the searches that have gained the most popularity with respect to the previous month) or the most Top Gaining Queries in a domain (that is, a selection of the top queries), whichever best reflects the zeitgeist.

http://www.google.com/intl/en/press/intl-zeitgeist.html#cr