An Arduino Leonardo (and compatible board) can operate as a USB keyboard or mouse device - and there's no shortage of projects demonstrating this for a variety of user-input options. One interesting example of this has been created by Instructables user "Kriegwesen" who has used an Arduino Leonardo to interface between a Nintendo Wii Nunchuk controller and a PC. The result of this is a Nunchuk Mouse, which operates in smooth manner and also incorporates replacements for the mouse button so it can be used for normal work or fun.
It really is interesting to see what you can do with Arduino and other interface objects, so click here to find out more about this one. And for more, we're on twitter and Google+, so follow us for news and product updates as well.
To get started with your own Arduino-USB mouse and keyboard projects - look no further than the Freetronics LeoStick. Apart from being one of the smallest Arduino Leonardo-compatibles on the market with USB, it also has an onboard RGB LED and piezo for sound and knock detection: