Mac: We've seen gesture-based mouse usage before, but SwiftRing simplifies the whole process by adding an easy to remember and intuitive control scheme.
SwiftRing is program specific and relies on holding down the Option key to work, so you won't find yourself accidentally swiping between web pages or saving documents you don't mean to. When holding down the key, you can move your mouse in on of four directions to create a gesture-based keystroke.
You can fully customise the options for each application, whether it's iTunes, Photoshop or your browsers. While this isn't handy for easy keystrokes like saving or tabbing across browsers, it might be a good replacement for more complicated keyboard shortcuts. [Oz editor: Except, er, you have to use the keyboard anyway. I don't quite get the appeal of this aspect.]