The latest version of the Chrome Canary app (that's Chrome's dev version, basically) includes a flag for turning on the notifications, as spotted by the Google Operating System blog. To turn on the notifications, you'll need to enable chrome://flags/#enable-google-now, then relaunch. Make sure you're signed in, and you should start seeing Google Now notifications in either the Windows tray or the Mac system bar.
It's pretty slick, and we're seeing all the same notifications as on our phone. Give it a whirl if you'd like.