Boxcar is an app for iOS and Mac (coming soon to Windows and Andriod) that allows you to receive instant push notifications from various apps. Remindor apps can send notifications to your Boxcar enabled devices if you set it up your Boxcar email in preferences.
Pushbullet is an app for Android, Chrome, and Firefox that allows you to send push notifications to/from your various devices. Remindor apps can send push notifications to a Pushbullet device that you choose. All you need to do is enter your api key in preferences.