Finally: an Arduino Voltmeter!
NOTE:
this is 100% OPEN SOURCE. Make money on it, for all I care. But please try not to do the exact copy. You could, but please don't.
Customize, too!
Ok. Enough babbling. Lets get to work.

YOU WILL NEED:
1 LCD
an Arduino
Jumpers, a lot (20-25)
A-B Usb cable
Something to measure. (Battery)
Soldering Iron
Rosin-Core Solder(Rosin Optional, but highly reccomended for a clean solder.

Step 1: Assemble and Wire.

First, please solder some male headers onto your LCD.
1. Heat Male headers
2. Add solder

Next, wire
Look at that picture below or read text.
LCD RS pin to digital pin 12
LCD Enable pin to digital pin 11
LCD D4 pin to digital pin 5
LCD D5 pin to digital pin 4
LCD D6 pin to digital pin 3
LCD D7 pin to digital pin 2
LCD R/W pin to ground
the d pins are usually 7-14 on lcd, D0 at pin 7

Step 2: CODE

Upload this code to your arduino:
/*
The circuit:
* LCD RS pin to digital pin 12
* LCD Enable pin to digital pin 11
* LCD D4 pin to digital pin 5
* LCD D5 pin to digital pin 4
* LCD D6 pin to digital pin 3
* LCD D7 pin to digital pin 2
* LCD R/W pin to ground
*/

// include the library code:
#include <LiquidCrystal.h> //Fire up the library.
int val = 0;

// What are the interface pins?
LiquidCrystal lcd(12, 11, 5, 4, 3, 2);

void setup() {
// set up the LCD's number of columns and rows:
lcd.begin(16, 2);
//Prints stuff.
pinMode(A0, INPUT);
lcd.setCursor(4,0); //Col 4, Row 0
lcd.print("ArVolt!"); //Change the name here.
lcd.setCursor(0,1); //Col 0, Row 1
lcd.print("Volts:"); //Unit goes here.
}

void loop() {
lcd.setCursor(7,1); //Col 7, Row 1
lcd.print(analogRead(0)/204.6); //Converts arduino's 0-1023 to 0.00-5.00
delay(100); //Slows down update rate, so you can see it.
}