The Blair Finch Project
Isle of Tune
This is a great beginning sequencer for students to create music electronically by building a city. They can view shared islands to see how people make their tunes, or create their own from scratch. We have found this to be an excellent tool for creating ostinatos and 3-part songs (they only give you 3 cars to "drive" the sounds around town). Kids love to play this, and can really work on ear training when adding in a melodic line without notation to rely on.
The basic idea of this is to create streets and fill them with bushes, houses, lights, and more. Each house or item placed on the island provides several sound choices, from percussion to melodies to sound effects. Each sound has its own volume control and limited rhythmic possibilities. It's easy to take a quarter note and turn it into eighth notes, but you have to be creative to add more complex rhythms.
The website is free and offers the opportunity to view and share islands, but the iPad app, though it costs a few dollars, it much easier to build with. They do offer discounts for educators to purchase multiple copies for classrooms.