As part of the updates to Google Search being introduced at I/O, Google Now is also getting a nice update with some new cards. Probably the most useful is a new voice reminders tool — you can tell your Android device to remind you to do something, and it'll bring up a quick dialog to let you set a specific time and then tap once to save the reminder. It's quite similar to adding reminders in iOS with Siri, though it appears there's no dedicated app to manage them.
Other new cards include public transportation data and a a list of new upcoming books, music, or movies you might be interested in (gleaned from your Google Play history, no doubt). There's also some new personal search features for those signed up with the Gmail search field trial — you can ask Google Now to send emails or search for specific photos you've saved in Google+. These features are available for Android now; the Google Search app update is live in Google Play.