Google Trends was originally released as a tool that let you see visual comparisons between search volume of keywords. That hasn't changed, but Google now also gives us the keys to more data about actual popularity of websites based on daily unique visitors.
This data is similar to what companies like Alexa already provides, but it's not ready to be a complete replacement yet. There is not as much data available through Google Trends as there is in other tools. Searches are currently limited to the domain level -- so blogs.zdnet.com would translate to zdnet.com before the search happens.
If you have a popular enough website to be included in these results from Google Trends, you may be disappointed to know that there is no way to remove your site if you wanted to. Google doesn't think that rule should apply to them though, as they have removed all their web properties -- searching for things like google.com or youtube.com comes back with nothing.
Also, another thing to keep in mind when looking at these stats is that they are all estimated -- not exact numbers according to their help file:
It's important to keep in mind that all results from Trends for Websites are estimated. Moreover, the data is updated periodically, so recent changes in traffic data may not be reflected. Finally, keep in mind that Trends for Websites is a Google Labs product, so it's still in its early stages of development and may therefore contain some inaccuracies.
With a little bit of work, this tool could become extremely interesting and useful. What do you think?