First (crooked and incomplete) implementation of aligning guidelines.

test/demo/aligning_guidelines.js

@@ -0,0 +1,103 @@
+function initAligningGuidelines(canvas) {
+  
+  var ctx = canvas.getContext(),
+      canvasHeight = canvas.getHeight(),
+      aligningLineOffset = 5,
+      aligningLineMargin = 4,
+      aligningLineWidth = 1,
+      aligningLineColor = 'rgb(0,255,0)';
+  
+  function drawVerticalLine(coords) {
+    drawLine(
+      coords.x + 0.5, 
+      coords.y1 > coords.y2 ? coords.y2 : coords.y1, 
+      coords.x + 0.5, 
+      coords.y2 > coords.y1 ? coords.y2 : coords.y1);
+  }
+  
+  function drawHorizontalLine(coords) {
+    drawLine(
+      coords.x1 > coords.x2 ? coords.x2 : coords.x1, 
+      coords.y + 0.5, 
+      coords.x2 > coords.x1 ? coords.x2 : coords.x1, 
+      coords.y + 0.5);
+  }
+  
+  function drawLine(x1, y1, x2, y2) {
+    ctx.save();
+    ctx.lineWidth = aligningLineWidth;
+    ctx.strokeStyle = aligningLineColor;
+    ctx.beginPath();
+    ctx.moveTo(x1, y1);
+    ctx.lineTo(x2, y2);
+    ctx.stroke();
+    ctx.restore();
+  }
+  
+  function isInRange(value1, value2) {
+    for (var i = value1 - aligningLineMargin, len = value1 + aligningLineMargin; i <= len; i++) {
+      if (i === value2) {
+        return true;
+      }
+    }
+    return false;
+  }
+  
+  canvas.onObjectMove = function(activeObject) {
+    
+    var canvasObjects = canvas.getObjects(),
+        activeObjectLeft = activeObject.get('left'),
+        activeObjectTop = activeObject.get('top'),
+        activeObjectHeight = activeObject.get('height'),
+        activeObjectWidth = activeObject.get('width'),
+        verticalLines = [ ],
+        horizontalLines = [ ];
+    
+    for (var i = canvasObjects.length; i--; ) {
+      
+      if (canvasObjects[i] === activeObject) continue;
+      
+      var objectLeft = canvasObjects[i].get('left'),
+          objectTop = canvasObjects[i].get('top'),
+          objectHeight = canvasObjects[i].getHeight(),
+          objectWidth = canvasObjects[i].getWidth();
+      
+      if (isInRange(objectLeft, activeObjectLeft)) {
+        verticalLines.push({ 
+          x: objectLeft, 
+          y1: (objectTop < activeObjectTop) 
+            ? (objectTop - objectHeight / 2 - aligningLineOffset) 
+            : (objectTop + objectHeight / 2 + aligningLineOffset), 
+          y2: (activeObjectTop > objectTop) 
+            ? (activeObjectTop + activeObjectHeight / 2 + aligningLineOffset) 
+            : (activeObjectTop - activeObjectHeight / 2 - aligningLineOffset) 
+        });
+        activeObject.set('left', objectLeft);
+      }
+      
+      if (isInRange(objectTop, activeObjectTop)) {
+        horizontalLines.push({
+          y: objectTop, 
+          x1: (objectLeft < activeObjectLeft) 
+            ? (objectLeft - objectWidth / 2 - aligningLineOffset) 
+            : (objectLeft + objectWidth / 2 + aligningLineOffset), 
+          x2: (activeObjectLeft > objectLeft) 
+            ? (activeObjectLeft + activeObjectWidth / 2 + aligningLineOffset) 
+            : (activeObjectLeft - activeObjectWidth / 2 - aligningLineOffset)
+        });
+        activeObject.set('top', objectTop);
+      }
+    }
+    
+    canvas.afterRender = function() {
+      for (var i = verticalLines.length; i--; ) {
+        drawVerticalLine(verticalLines[i]);
+      }
+      for (var i = horizontalLines.length; i--; ) {
+        drawHorizontalLine(horizontalLines[i]);
+      }
+    };
+  };
+  
+  
+}

test/demo/demo.js

@@ -333,4 +333,8 @@
   
   initCenteringGuidelines(canvas);
   
+  if (document.location.search.indexOf('align') > -1) {
+    initAligningGuidelines(canvas);
+  }
+  
 })(this);

test/demo/index.html

@@ -151,6 +151,7 @@
       </div>
     </div>
     <script src="centering_guidelines.js" type="text/javascript"></script>
+    <script src="aligning_guidelines.js" type="text/javascript"></script>
     <script src="demo.js" type="text/javascript"></script>
   </body>
 </html>