Windows/Mac: Mobile Mouse is a remote control tool for Windows and Mac computers that isn't just a mobile trackpad and keyboard but a remote control with a library of specific application interfaces and an application launcher too.
Mobile Mouse goes beyond the simple touchscreen-as-touchpad design by incorporating additional features such as a keyboard (the keyboard on your iPhone or iPad becomes a wireless keyboard), application launching on the remote computer, accelerometer-based mouse control, and customised media controls (the last two features are only available in the $US2.99 Pro version). If you use a lot of media apps on your computer you'll appreciate the huge library of custom media controls available in the Pro version; they've created a keymap for controlling different apps with specific layouts for the major functions of that application.
Mobile Mouse is available as a free and Pro version and requires you install a small helper application on your Mac or Windows PC. Visit the the link for more information.