Roll out begins today, should be available for all in the next few weeks
We've seen Google Now for Chrome on the desktop for folks using beta builds, but starting today it's pushing out to all users. You won't need to fiddle with any settings or flags or do any sort of wizardry, but you will need to be on the latest version of Chrome — which you should be anyway for security reasons. Once Google Now for Chrome is available for you, you use it just like you would on the phone, with reminders and cards in your tray, and voice commands through the Google website. For more information, see this Google help topic page.
Source: +Google Chrome