A free Google service that provides charts that show how often words, phrases and topics have been searched for over time. Using Google Trends you can compare up to five topics at one time and also see how often those topics have been mentioned in news stories and find out in which geographic regions the topics have been searched for the most. Google Trends also offers "Hot Trends" which is a snapshot of top 40 fastest-rising search queries in the United States.
Not only that... But use Google Trends as well. It is a gold mine for new keywords that are not showing up in teh other keyword tools...
Google trends shows its current demand by comparing with last months, It keep on changing and important point while selecting keywords