A great digital circuit simulator! I have been using it for over a year in the digital electronics class I teach at the University of Brasilia, and I highly recommend it.
Great package. Has a few bugs however, and it is no longer updated, so logisim evolution might be worth looking into.
Excellent package. I used it for 3 years to teach parts of Computer Architecture, but it can also be used to show many of the operations that happen 'under the hood' on a programming course.