Keywords Everywhere is an SEO extension that focuses on keyword research. After you install it on your toolbar, it will show you monthly search volume for any keyword that you look up. You can also see competition data and CPC on more than 10 popular websites.
There is a free and a paid version of this extension. The free version has a trend chart, People Also Search, trending and long-tail keywords. It also shows you the 5000 keywords for all pages.
If you get the paid version you can see monthly search volume, competition trends and volume and a historical chart with data from 2004.
Keywords Everywhere brings you keyword data from Google, YouTube, Amazon and other popular sites.