Based on Google search results of course, this Google tool shows you relevant keyword ideas and their Adwords competition for your website.   Related posts: Get New Keyword Ideas Stats On PPC Ads Monthly Keyword Search Volume

Wikipedia is widely popular – and has made its visitor stats available: Wikirank shows popular pages on Wikipedia which may suggest a trend on the web, or just on WIkipedia? How will you use it?   Related posts: Show Related Posts in WordPress Finding Keyword From Questions Keyword Trends