Microsoft's Universal Foldable Keyboard does a nice magic trick: Folded up, it's only 0.5 x 4.9 x 5.8 in. and weighs 6 oz., but open its single hinge and you've got an 11.6-in.-long keyboard.
Once you've paired it with a phone or tablet, the 75-key device automatically connects via Bluetooth 4.0 when opened and disconnects when closed. You can easily move between two systems as well; the keyboard works with Windows computers and phones as well as Android, iOS and Mac systems.
The keyboard does have couple of quirks, like a split space bar and enlarged "T," "G," "H" and "N" keys -- the result of the diagonal stacking of the keys and the hinge, which sits at a right angle. It takes some getting used to, but I had no real problems with it. In addition, unlike some other mobile keyboards, it lacks a place to prop up a phone or tablet.
However, when folded, the Universal Foldable Keyboard is about the size of a paperback book, making it the perfect travel companion for the touch typist who wants to work on a mechanical keyboard.