Google has launched in the past like 2 years ago Google trends for keywors but now Google is after compete.com, alexa.com, comscore.com and hitwise.com by showing website traffic and trends ranking filtered out by

http://trends.google.com/website
keywords
competition
other visited sites
geographical traffic and trend
yearly, monthly and last 30 days trend

Probably you would say it is the best tool out there but watch out since Google is hidding and not showing Data for www.google.com, youtube.com, gmail.com, all other Google .tld websites.

GOOGLE IS saying that DATA IS UNAVAILABLE for YOUTUBE.com Google.com Gmail.com, WHY_?

Good sample for site trends as for barackobama.com VS hillaryclinton.com

http://trends.google.com/websites?q=barackobama.com%2Chillaryclinton.com&geo=all&date=all&sort=0

People who visited BarackObama.com comes to his site by searching for>
Also searched for
1. fight smears
2. obama president
3. barack obama
4. obama campaign
5. barackobama
6. obama
7. barack obama website
8. fightthesmears
9. barak obama
10. obama website

Also visited
1. fightthesmears.com
2. obama.com
3. barakobama.com
4. johnmccain.com
5. democrats.org
6. hillaryclinton.com
7. talkingpointsmemo.com
8. tnr.com
9. fivethirtyeight.com
10. mydd.com

But people who came to Hillaryclinton.com website searched for>
Also searched for
1. hillary clinton
2. hilary clinton website
3. hillary
4. hilary clinton
5. hillary clinton president
6. clinton
7. barack obama
8. obama
9. hilary
10. obama president

Also visited
1. hillaryhub.com
2. hilaryclinton.com
3. hillarystore.com
4. hillary.com
5. thehillaryiknow.com
6. youpolls.com
7. democrats.org
8. barackobama.com
9. taylormarsh.com
10. hillaryclintonforum.net