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);
 		
  	}
}	

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) !