Arduino 1.0-compatible nunchuck example based on http://www.windmeadow.com/node/42

nunchuck

#include <Wire.h>
#include <string.h>

#undef int
#include <stdio.h>

uint8_t outbuf[6];		// array to store arduino output
int cnt = 0;
int ledPin = 13;

void
setup ()
{
  Serial.begin (19200);
  Serial.print ("Finished setup\n");
  Wire.begin ();		// join i2c bus with address 0x52
  nunchuck_init (); // send the initilization handshake
}

void
nunchuck_init ()
{
  Wire.beginTransmission (0x52);	// transmit to device 0x52
  Wire.write (uint8_t(0x40));		// sends memory address
  Wire.write (uint8_t(0x00));		// sends sent a zero.  
  Wire.endTransmission ();	// stop transmitting
}

void
send_zero ()
{
  Wire.beginTransmission (0x52);	// transmit to device 0x52
  Wire.write (uint8_t(0x00));		// sends one byte
  Wire.endTransmission ();	// stop transmitting
}

void
loop ()
{
  Wire.requestFrom (0x52, 6);	// request data from nunchuck
  while (Wire.available ())
    {
      outbuf[cnt] = nunchuk_decode_byte (Wire.read ());	// receive byte as an integer
      digitalWrite (ledPin, HIGH);	// sets the LED on
      cnt++;
    }

  // If we recieved the 6 bytes, then go print them
  if (cnt >= 5)
    {
      print ();
    }

  cnt = 0;
  send_zero (); // send the request for next bytes
  delay (100);
}

// Print the input data we have recieved
// accel data is 10 bits long
// so we read 8 bits, then we have to add
// on the last 2 bits.  That is why I
// multiply them by 2 * 2
void
print ()
{
  int joy_x_axis = outbuf[0];
  int joy_y_axis = outbuf[1];
  int accel_x_axis = outbuf[2] * 2 * 2; 
  int accel_y_axis = outbuf[3] * 2 * 2;
  int accel_z_axis = outbuf[4] * 2 * 2;

  int z_button = 0;
  int c_button = 0;

 // byte outbuf[5] contains bits for z and c buttons
 // it also contains the least significant bits for the accelerometer data
 // so we have to check each bit of byte outbuf[5]
  if ((outbuf[5] >> 0) & 1)
    {
      z_button = 1;
    }
  if ((outbuf[5] >> 1) & 1)
    {
      c_button = 1;
    }

  if ((outbuf[5] >> 2) & 1)
    {
      accel_x_axis += 2;
    }
  if ((outbuf[5] >> 3) & 1)
    {
      accel_x_axis += 1;
    }

  if ((outbuf[5] >> 4) & 1)
    {
      accel_y_axis += 2;
    }
  if ((outbuf[5] >> 5) & 1)
    {
      accel_y_axis += 1;
    }

  if ((outbuf[5] >> 6) & 1)
    {
      accel_z_axis += 2;
    }
  if ((outbuf[5] >> 7) & 1)
    {
      accel_z_axis += 1;
    }

  Serial.print (joy_x_axis, DEC);
  Serial.print ("\t");

  Serial.print (joy_y_axis, DEC);
  Serial.print ("\t");

  Serial.print (accel_x_axis, DEC);
  Serial.print ("\t");

  Serial.print (accel_y_axis, DEC);
  Serial.print ("\t");

  Serial.print (accel_z_axis, DEC);
  Serial.print ("\t");

  Serial.print (z_button, DEC);
  Serial.print ("\t");

  Serial.print (c_button, DEC);
  Serial.print ("\t");

  Serial.print ("\r\n");
}

// Encode data to format that most wiimote drivers except
// only needed if you use one of the regular wiimote drivers
char
nunchuk_decode_byte (char x)
{
  x = (x ^ 0x17) + 0x17;
  return x;
}

No modifications to the Wire library were needed. 3.3V source on Arduino Uno seemed to work.