The fact that Microsoft has somehow missed the tablet revolution entirely, a segment it basically created with the Tablet PC in 2001, is the cruelest, most perfect distillation of the company's execution problem. The first Tablet PCs failed to distinguish themselves sufficiently from laptops, running essentially the same operating system (Windows XP) with less power and memory and no new, compelling user experience to speak of. Since the iPad took the mantle in 2010, Microsoft has been trying to catch up. 

But before tablets saw a resurgence with competitors, Redmond had its own re-imagining of the device up its sleeve. The Courier was a tablet unlike any that came before or since. It adopted a booklet form factor, with two vertically oriented touch screens joined by a hinge. It relied on a stylus for input and was more like a digital journal than the iPod Touch-like devices we're used to today. Because of the way you opened and closed it, the Courier also naturally served as an eReader, and was usually shown with the kind of leather case that you might find on a Moleskine.

Images and video of the Courier first hit the web in early 2010, and naturally, speculation centered on it serving as a competitor to Apple's first iPad. But it wasn't to be. Microsoft announced that it had no plans to develop the Courier as a product, baffling observers who had been intrigued by the tablet's unique concepts. The company's bid for taking back the tablet instantly vaporized.