Android only: SMS Popup gives you the best of both smartphone worlds—the immediacy of the iPhone's pop-up SMS notifications, along with the utility of being able to reply with preset messages, your voice, or text, all from a small window.
Install SMS Popup, then head into its settings. You'll be able to set when it comes up, and whether it should mark messages as "Read" once it does pop up. You can also set which three buttons you'd like to have available when you see a message—close, reply, quick reply, delete, or speech-to-text, to have your message read to you. The Quick Reply is key here—you can ping back with your voice, with text, or a pre-written message you enter in the settings, like so:
SMS Popup is a free download for Android phones only. Grab it using the direct QR download code at left.