Example of using Charlieplexing [http://en.wikipedia.org/wiki/Charlieplexing] to control 6 LEDs with an arduino.

gistfile1.c

/** Charlieplexing 6 LEDs with Arduino Uno     **/
/** Jeff Larkin [http://github.com/jefflarkin] **/
/*
 * Wiring Diagram:
 *
 * pin 2 ===== led0+, led1- ===== led4+, led5-
 * pin 4 ===== led1+, led0- ===== led2+, led3-
 * pin 6 ===== led3+, led2- ===== led5+, led4-
 *
 */
int pins[] = {2,4,6};

void setup()
{
  for (int i = 0; i < 3; i++)
  {
    pinMode(pins[i],INPUT);
  }
}

void loop()
{
  for(int i = 0; i < 6; i++)
  {
    lightLED(i);
    delay(250);
  }
}

void lightLED(int i)
{
  switch(i) 
  {
  case 0:
    pinMode(pins[0],OUTPUT);
    pinMode(pins[1],OUTPUT);
    pinMode(pins[2],INPUT);
    digitalWrite(pins[0],HIGH);
    digitalWrite(pins[1],LOW);
    break;
  case 1:
    pinMode(pins[0],OUTPUT);
    pinMode(pins[1],OUTPUT);
    pinMode(pins[2],INPUT);
    digitalWrite(pins[1],HIGH);
    digitalWrite(pins[0],LOW);
    break;
  case 2:
    pinMode(pins[0],INPUT);
    pinMode(pins[1],OUTPUT);
    pinMode(pins[2],OUTPUT);
    digitalWrite(pins[1],HIGH);
    digitalWrite(pins[2],LOW);
    break;
  case 3:
    pinMode(pins[0],INPUT);
    pinMode(pins[1],OUTPUT);
    pinMode(pins[2],OUTPUT);
    digitalWrite(pins[1],LOW);
    digitalWrite(pins[2],HIGH);
    break;
  case 4:
    pinMode(pins[0],OUTPUT);
    pinMode(pins[1],INPUT);
    pinMode(pins[2],OUTPUT);
    digitalWrite(pins[0],HIGH);
    digitalWrite(pins[2],LOW);
    break;
  case 5:
    pinMode(pins[0],OUTPUT);
    pinMode(pins[1],INPUT);
    pinMode(pins[2],OUTPUT);
    digitalWrite(pins[0],LOW);
    digitalWrite(pins[2],HIGH);
    break;
  }
}