(Updated July 2012)
Google Insights – With Google Insights for Search, you can compare search volume patterns across specific regions, categories, time frames and properties.
Google Insights Support – Showcases some different ways of using Google Insights for Search.
Google Trends – Google Trends is a public web facility of Google Inc., about Google Search, that shows how often a particular search-term is entered relative to the total search-volume across various regions of the world. The tool also gives you an idea of the top 100 keyword searched for everyday.
Quantcast – Free direct audience measurement for all website owners including traffic, demographics, business, lifestyle, interests and more.
TrendWatching – Delivering trends, insights and related hands-on innovations.