Do you want to build a serial or USB based joystick? Serial would be relatively easy to build, you just need to add something like a MAX232 or equivalent chip to translate from the Atmega's 5V to the required +- 12V as required for serial communication. This is probably a dead end though since serial ports are quite scarce on modern computers.You could build a USB based mouse by implementing the HID interface for a mouse using the V_USB library (http://www.obdev.at/products/vusb/index.html). An Arduino port of this library can be found at http://code.google.com/p/vusb-for-arduino/. This will be a little bit more tricky and will require some internet searching for examples to adapt. Some examples can be found at http://www.obdev.at/products/vusb/prjhid.html but are not specifically adapted to the Arduino environment.