Apple's official iOS UI guidelines. Will be pretty important going forward for iOS development, as this is some of the stuff that Apple looks for when approving Apps.
A lot of people have voiced their concerns over Microsoft's use of "touch-based" UI with Windows 8. In an attempt to gauge how the "average" desktop user will engage and react to Windows 8's Metro design, lockergnome had his dad sit down and use Windows 8 without any previous instruction.