Though the Xbox One itself isn't available until Friday, that doesn't mean you can't download the Xbox One SmartGlass app now and, I don't know, maybe stare at it for the next three days until release.
"Xbox One SmartGlass is the perfect companion app for your Xbox One, both in your living room and on the go. Connect and control your Xbox One. Stay connected with the games and gamers you like. Add a new dimension to your console entertainment with the mobile device you already own."
The Xbox One SmartGlass app will also allow you to use it as a remote control or keyboard when connected to your system. It also reportedly features better connectivity than the old app did. When you aren't connected it still lets you browse achievements and the marketplace, and even watch some clips you had on the DVR.
It's available now for Android, Windows Phone, and Windows 8 (warning: this URL will probably open the Store within Windows 8 itself if you click on it). An iOS version is coming soon.