updated 10:27 am EST, Thu January 16, 2014
Currently tied to mobile devices
The newest build of Chrome Canary -- Google's experimental browser -- adds preliminary Google Now support. Once the option is on, Canary will push notifications to the taskbar in Windows, or the menu bar in OS X. There are some caveats, however -- users must run Google Now on a mobile device first, and location-based cards are tied to that device, rather than the computer Canary is on.
Once Canary testing is complete, Now support will presumably carry over to Chrome betas, and then a polished final release. The feature is used to push predictive information based on location, Gmail and Calendar content, and recent Google searches. Gmail messages with package and flight numbers, for instance, will trigger tracking info in Now; Google Calendar appointments will signal when a person needs to leave to make it to their destination on time.
Now was originally exclusive to Android devices. It has since migrated to iOS, though, via the Google Search app. Unlike Android, iOS users must open Google Search to see cards. Push notifications are used to compensate.