Avoid "weight gainers" that are packed with sugar. The weight you will gain will be fat. If you want lean muscle gain, eat pleanty of calories, and focus on protein. I have always supplemented with the same protein powder as I recommend to my clients, and it has no sugar, only natural ingreadient and high biological value protein sources. For weight gain purposes, I add extra healthy fats to it, including raw eggs (only from organic free range chickens).
-Adam