Google Trends

http://www.google.com/trends

Google Trends compares the popularity of search terms over a period of time and lists what's most popular every day and each year. Last year, Google built Google Insights for Search, a beefier version aimed at helping advertisers use search advertising more effectively. Microblogging site Twitter followed up this year with its own application that reports the hottest topics being discussed by its users at any given moment.