You’re probably familiar with regular Google Trends, which lets you see trends in how people search for difference phrases such as full moon or skiing vs. swimming. Here’s the graph of how often people search for “full moon,” for example: 
Those spikes correspond nicely to when full moons actually happen. Now you can do similar fun things with sites. Here’s a simple example with my site, mattcutts.com:
As you can see, Trends will try to estimate the number of visitors to my site over time. (The product is free, but you have to sign in if you want to get estimated numbers — otherwise you see the graph but not the number estimates.) There’s other good info too, though. See the graph below, for example:
This is saying that people who visited mattcutts.com also visited searchmarketingexpo.com and sphinn.com. You can “surf” related sites just by clicking around. You can also see what else people searched for. And you can even enter in two sites, separated by commas, to compare the estimated number of visitors between the sites:
It’s a lot of fun, especially if website metrics is your cup of tea. You can read the official blog post or Barry Schwartz did a write-up as well. The comments are pointing out that some sites might not have much/any data. I think that that’s mainly because there’s a minimum threshold of traffic before Trends is willing to show statistics for a site — bear in mind that this is a launch on Google Labs. But you can still do some fun analysis with Trends for Websites, even though it’s in Google Labs. And it is free, so give it a try.
http://www.mattcutts.com/blog/type/googleseo/
