Google Trends for Websites is an excellent tool which allows you to track and view unique visitors to a website or multiple websites at the same time.
You can also have the results ranked by individual website as well as viewing trend records by different variables like region or specific timeframe.
Google Trends lists similar websites that are often searched as well as related high volume search terms that have been recently searched.
This tool not only makes it easy to see your unique visitor trends, but it allows you to see if your SEO efforts are yielding positive results.
Can be used as a research tool to compare similar sites and view visitor trends such as in the attached image below comparing technology vendors and showing spikes in traffic at similar time frames (Christmas shopping spike?).