Google Chrome Canary is the cutting-edge version of the Google Browser. At the moment, you can download Google Chrome Stable, Google Chrome Beta, Google Chrome Developer versions and now, Google Chrome Canary which is the Alpha/pre-Alpha version of the browser.
This means that anything that appears in Google Canary, will eventually probably appear in both the developer and stable versions, so if you want to stay at the very cutting edge of Google Chrome, Canary is what you need. The good news is, when you install Google Canary, it won't install itself over any other version you've got installed, so you can test it to your heart's content.
However, since its completely untested and designed for those that want to help Google develop the next version of Chrome, you'll find it very unstable and unreliable at times and be warned that it will send anonymous usage stats back to Google.
If you want to see the cutting edge latest in the development of Chrome, Google Chrome Canary is your answer.