This website offers awesome wind prediction in the form of simple animation. You can see what the wind is to be like on the whole planet and at various altitudes. And not just the wind, you can view temperature, pressure, humidity, clouds, precipitation and also waves.

Apart from normal weather predictions, the czech hydrometeorological institute website contains a lot of aviation weather predictions too.

If you like to fly with real world weather and you fly in the USA, this is the right place to get complete weather picture.