Google just released their "website trends" data out to the world under their trends product.
Basically, google uses the data from opt-in google analytics accounts, their click through data and who-knows what-else to estimate traffic.
Up until now, I've been using compete.com to do competitive research. You can enter in your (clients) sites and the competition, and see the traffic stats. Now, this data is a bit inaccurate, but it does give you a really good idea of where you are holding. Additionally, you can get the keywords that are driving the traffic to those sites - forreally cheap. It's alot better than paying hitwise $695 for probably more questionable data.
Well, now Google trends shows alot of easy to use data in a simple format.
#1 - traffic stats. If you are logged in, you can see traffic stats for each of the sites
#2 - top keywords searched
#3 - locations in the world, and country of visitors for each site.
#4 - other sites that visitors of these websites visit. This is really sweet - basically, you get to see who all of your competitors are!
Some other articles on google trends: