Per Matt Cutts. Wow this is fun!!! I highly suggest you give it a try right now. I knew there were lots of paid tools like this, but a free one by Google is good times.
Reminds of Yahoo’s Buzz Index but way cooler – comparisons between terms, regions, etc. Good times.
Update: Comparison of “housing boom” versus “housing bubble” – my interest was peaked after reading a post on Bubble Meter that was comparing the relative popularity of each phrase by looking at results in the SERPs. Google Trends looks at search volume.