This is a PS script to make Sprite Sheets out of layers. Modified from http://www.garagegames.com/community/blogs/view/11527

layers_to_sprite_sheet.js

// Put this file in Program Files\Adobe\Photoshop\Presets\Scripts\
// In PhotoShop menu File > Automate > Scripts: layersToSprite.js

// Arrange layers into a sprite sheet. 

if (documents.length > 0) 
{
	
	// --------------------------
	docRef = activeDocument;    
	var activeLayer = docRef.activeLayer;

	numLayers = docRef.artLayers.length; 	
	var cols = docRef.width;
	
 	var spriteX = docRef.width;
 	
	// put things in order
	app.preferences.rulerUnits = Units.PIXELS;
	
	// resize the canvas
 	newX = numLayers * spriteX;
 	
 	docRef.resizeCanvas( newX, docRef.height, AnchorPosition.TOPLEFT );
 	 	
	// move the layers around
 	for (i=0; i < numLayers; i++)
 	{ 	
 		docRef.artLayers[i].visible = 1;
 		
  		var movX = spriteX*i;
  		
 		docRef.artLayers[i].translate(movX, 0);
 		
  	}
}	

Nice work. Thank you, JesseFreeman.

An improved version which takes sprite count per row as input to break sheet into columns, counts empty spaces, is background aware and allows putting sprites in reverse order:

(latest version in this gist)

// Put this file in Program Files\Adobe\Photoshop\Presets\Scripts\
// In PhotoShop menu File > Automate > Scripts: Layers To Sprite Sheet.js

// Arrange layers into a sprite sheet. 

if (documents.length > 0) {
    var docRef = activeDocument;

    var numLayers = docRef.artLayers.length;
    var cols = docRef.width;

    var spriteX = docRef.width;
    var spriteY = docRef.height;

    app.preferences.rulerUnits = Units.PIXELS;

    var hasBackground = false;
    var layerNumbers = [];
    for (i = 0; i < numLayers; i++) {
        if (docRef.artLayers[i].isBackgroundLayer) {
            docRef.artLayers[i].visible = false;
            hasBackground = true;
        } else {
            layerNumbers.push(i);
        }
    }

    var numSprites = layerNumbers.length;

    var rowSize = prompt("How many sprites per row?", "8", "Configure stylesheet");

    var rowCount = Math.ceil(numSprites / rowSize);
    var emptySpaces = (rowSize * rowCount) - numSprites;

    while (rowSize && !confirm("Is " + rowSize + "x" + rowCount + " sheet OK? There will be " + emptySpaces + " empty spaces.")) {
        rowSize = prompt("How many sprites per row?", "8", "Configure stylesheet");
        rowCount = Math.ceil(numSprites / rowSize);
        emptySpaces = (rowSize * rowCount) - numSprites;
    }

    if (rowSize) { // cannot return from function in photoshop :/
        var reverse = confirm("Use reverse order? Photoshop creates new sprites on top, with reverse order first sprite will be the most bottom layer");
        if (reverse) {
            layerNumbers.reverse();
        }

        var newX = rowSize * spriteX;
        var newY = rowCount * spriteY;

        docRef.resizeCanvas(newX, newY, AnchorPosition.TOPLEFT);

        var startOffset = hasBackground ? 2 : 1;
        startOffset += emptySpaces;

        for (i = 0; i < rowSize; i++) {
            for (j = 0; j < rowCount; j++) {
                var layer = rowSize * j + i;
                if (layer >= numSprites) {
                    continue;
                }
                layer = layerNumbers[layer];
                try {
                    if (docRef.artLayers[layer].isBackgroundLayer) {
                        docRef.artLayers[layer].visible = 0;
                    } else {
                        docRef.artLayers[layer].visible = 1;

                        var movX = spriteX * i;
                        var movY = spriteY * j

                        docRef.artLayers[layer].translate(movX, movY);
                    }
                } catch (e) {
                    alert("Layer " + layer + " at row:col " + j + ":" + i + " failed: " + e.message);
                }
            }
        }
    }
} 

Thank you @jessefreeman and @jsonacob (I was needed a vertical solution) !