Google Trends allows you you to check and compare how popular a particular search term, or collection of search terms, is. Very useful when deciding which are the most popular terms in a selection you are looking to use in your campaigns. A while back Google extended the functionality to allow comparison of websites. Google Trends for Websites shows the number of Unique Daily Visitors to a web site and allows you to compare web sites too. Simply enter the site URLs separate by commas.  Useful to see how you are doing relative to your possible peers and competitors. Another couple of very useful additional pieces of information are also provided – a list of other sites that people viewing your chosen sites have visited AND the the top terms they have used. You can sort all this lot by region and vary the period of interest eg last mth, last 12 months, etc. The main drawback I have found is the lack of data on smaller sites and less popular search terms. Still, valuable data to help you get and stay on the right track.