We've seen a few different magic mirror projects using a Raspberry Pi, but in the newest issue of MagPi they have put together what might as well be the definitive magic mirror guide as it's easily the simplest one to make for yourself.
Like any of these magic mirror projects, the most difficult part is the actual construction. To make this, you'll need to deconstruct an old monitor or TV, build a wooden frame around it, cram a Raspberry Pi inside that, then mount a two-way mirror over it.
The good news is that once you get past the difficulty of assembling the mirror, MagPi has made it so you can install all the software you need to actually run this on a Raspberry Pi with a single line of code. Once its installed, the Pi powers a dashboard that includes the date, temperature, calendar, a news feed and a message. You can then customise that info with your own data, or better yet, install a third-party module like one that displays the newest XKCD comic or an RSS feed. This is probably as simple as it's going to get to make your own magic mirror, so if you've been thinking about doing it, do it now.
MagPi Issue 54 (PDF)