Photoshop script for adding measurements to your mockups, based on Pixel Measure v0.04 by Nikolaj Selvik (https://code.google.com/p/pixelmeasure/)

Pixel Measure.jsx

/*
Pixel Measure v0.04 - Photoshop script for adding pixel measurements to your mockups
Copyright (C) 2009 Nikolaj Selvik

This program is free software; you can redistribute it and/or
modify it under the terms of the GNU General Public License
as published by the Free Software Foundation; either version 2
of the License, or (at your option) any later version.

This program is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
GNU General Public License for more details.

You should have received a copy of the GNU General Public License
along with this program; if not, write to the Free Software
Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301, USA.
*/

//=====================================================
// ADDED: DIMENSION MEASUREMENT UNITS
// by @alingham
// Adding Measurement options into the script. Still produces the dimension line in pixels, but changes the label to be in 
// Inches or Millimeters or Pixels (default) depending on the original unit of measurement set on the ruler in Photoshop. 
//=====================================================

var originalUnit = preferences.rulerUnits;
preferences.rulerUnits = Units.PIXELS;

app.displayDialogs = DialogModes.NO;

if(validateState())
{
	app.activeDocument.suspendHistory("Pixel Measure", "createMeasure();");
}


function validateState()
{
	if (app.documents.length == 0)
	{
		alert("No document open");
		return false;
	}
	 
	if(!hasSelection(app.activeDocument))
	{
		alert("Please make a selection to measure");
		return false;
	}
	
	return true;
}	


function createMeasure()
{
	var docRef = app.activeDocument;
	var selRef = docRef.selection;
	var mainLayerSet;
	
	// =======================================================
	// Create "Pixel Measures" LayerSet if it doesn't already exist
	// =======================================================
	try
	{
		mainLayerSet = docRef.layerSets.getByName("Pixel Measures");
	}
	catch(error)
	{
		mainLayerSet = docRef.layerSets.add();
		mainLayerSet.name = "Dimensions";
	}

	// =======================================================
	// Create Measurement LayerSet
	// =======================================================
	var layerSetRef = mainLayerSet.layerSets.add() 
	var linesLayerRef = layerSetRef.artLayers.add();

	// =======================================================
	// Draw Lines
	// =======================================================
	var x1 = selRef.bounds[0].value;
	var y1 = selRef.bounds[1].value;
	var x2 = selRef.bounds[2].value;
	var y2 = selRef.bounds[3].value;

	selRef.deselect();

	var width = x2 - x1;
	var height = y2 - y1;

	var horizontal = width > height;

	if(horizontal)
	{
		drawLine(x1,y1,x1,y1+10);
		drawLine(x2-1,y1,x2-1,y1+10);
		drawLine(x1,y1+5,x2-1,y1+5);
	}
	else
	{
		drawLine(x1,y1,x1+10,y1);
		drawLine(x1,y2-1,x1+10,y2-1);
		drawLine(x1+5,y1,x1+5,y2-1);
	}


	// =======================================================
	// Draw Text
	// =======================================================
	var textLayerRef = layerSetRef.artLayers.add();
	textLayerRef.kind = LayerKind.TEXT;
	var textItemRef = textLayerRef.textItem;
	
	//Get Measurement from Ruler, and Resolution from Document
    if (document === undefined) {
            var document = app.activeDocument;
    }

        var resolution = document.resolution;
	if (originalUnit == "Units.INCHES") {
		var dimensionwidth = width/resolution;
		var dimensionheight = height/resolution;
		var unit = "in";
		var dp = 2;
	}
	else if (originalUnit == "Units.MM") {
		var dimensionwidth = (width/resolution)*25.4;
		var dimensionheight = (height/resolution)*25.4;
		var unit = "mm";
		var dp = 0;
	}
	else if (originalUnit == "Units.CM") {
		var dimensionwidth = (width/resolution)*2.54;
		var dimensionheight = (height/resolution)*2.54;
		var unit = "cm";
		var dp = 1;
	}
	else {
		//default pixels (PX)
	    var dimensionwidth = width;
	    var dimensionheight = height;
	    var unit = "px";
	    var dp = 0;
	}
	if(horizontal)
	{
		textItemRef.contents = dimensionwidth.toFixed(dp) + " " + unit;
		textItemRef.justification = Justification.CENTER;
		textItemRef.position = Array(Math.floor(x1 + (width/2)),y1+21);
	}
	else
	{
		textItemRef.contents = dimensionheight.toFixed(dp) + " " + unit;
		textItemRef.position = Array(x1+15,Math.floor(y1 + 4 + (height/2)));
	}

	layerSetRef.name = textItemRef.contents;

	textItemRef.color = app.foregroundColor;
	textItemRef.font = "ArialMT";

	// =======================================================
	// Reset
	// =======================================================
	app.preferences.rulerUnits = originalUnit;
}


function drawLine(x1,y1,x2,y2)
{
	var pointArray = new Array();

	var pointA = new PathPointInfo();
	pointA.kind = PointKind.CORNERPOINT;
	pointA.anchor = Array(x1, y1);
	pointA.leftDirection = pointA.anchor;
	pointA.rightDirection = pointA.anchor;
	pointArray.push(pointA);

	var pointB = new PathPointInfo();
	pointB.kind = PointKind.CORNERPOINT;
	pointB.anchor = Array(x2, y2);
	pointB.leftDirection = pointB.anchor;
	pointB.rightDirection = pointB.anchor;
	pointArray.push(pointB);

	var line = new SubPathInfo();
	line.operation = ShapeOperation.SHAPEXOR;
	line.closed = false;
	line.entireSubPath = pointArray;

	var lineSubPathArray = new Array();
	lineSubPathArray.push(line);

	var linePath = app.activeDocument.pathItems.add("TempPath", lineSubPathArray);
	linePath.strokePath(ToolType.PENCIL, false);
	app.activeDocument.pathItems.removeAll();
}

function hasSelection(doc)
{
	var res = false;
	var as = doc.activeHistoryState;
	doc.selection.deselect();
	if (as != doc.activeHistoryState)
	{
		res = true;
		doc.activeHistoryState = as;
	}
	return res;
}

//=====================================================
// ADDED: DIMENSION MEASUREMENT UNITS
// by @alingham
// Adding Measurement options into the script. Still produces the dimension line in pixels, but changes the label to be in
// Inches or Millimeters or Pixels (default) depending on the original unit of measurement set on the ruler in Photoshop.
//=====================================================

Thank you, very useful tool.