The Chrome OS notification center -- already in the Windows releases of Chromium and Chrome Canary -- is in the process of being ported to the Mac version of Chromium, says
Google's Francois Beaufort. The Chromium evangelist points to a logged
Chromium issue titled "[Mac][MC] Implement notification updates and relayouts." He comments that the Mac version of the notification center is still "under heavy development," but that it can already be toggled on by running Chrome with an "--enable-rich-notifications" switch.
The port is thought
to be a precursor to Google Now arriving on Macs, first via Chromium, and then Chrome itself. The service can already be used in Chrome OS or Windows by modifying a URL reference, but is otherwise only available through Android and iOS. Notifications are necessary since the point of Now is to push information "cards" to a person before they need to run a Google search.