Adafruit: 2-Axis Thumb Joystick
The Adafruit: 2-Axis Thumb Joystick uses 2 10 kOhm pots which are adjusted by moving the joystick. The joystick and development board can be purchased from the Adafruit Website. The joystick is simplistic enough that Adafruit does not provide a datasheet.
From the Manufactorer: This mini-kit makes it easy to mount a PSP/Xbox-like thumb joystick to your project. The thumbstick is an analog joystick - more accurate and sensitive than just 'directional' joysticks - with a 'press in to select' button. Since it's analog, you'll need to analog reading pins on your microcontroller to determine X and Y. Having an extra digital input will let you read the switch.
Inputs and Outputs
The Adafruit joystick takes a supply voltage (Vs) of up to 5V. The analog outputs have a direct correlation to the resistance observed by the analog joystick. At the resting position, the resistance in both axis is at its middle resistance or 5k. At either extreme of the range of motion, there is either enough resistance for the analog inputs to register a 0 or so little resistance that the supply line to the analog input looks open.