The Music Tech Teacher is a site full of resources for elementary music teachers.
One really great page on this site is the Instrument Families page. This is a fabulous resource that has picture examples of every instrument from the four main instrument families as well as an audio clip of each being played. This would be a great way to show students the difference between two different but similar instruments. As an added bonus more than one instrument at a time can be played for a combination of sounds.