Google Trends for Websites is a web-based service where you can compare the visitor statistics of websites.
The data displayed is daily unique visitors (after logging in, you can see the numbers that graphs represent) and currently, up to 5 websites can be compared.
Google Ad Planner provides you information about websites by simply entering demographics and sites associated with your target audience. This is a tool to be used when you are planning or searching for websites to advertise.
Ad Planner, currently, can be used by invitation only. You can apply for it here.
More info can be found at Inside Adwords blog.