I love logisim. I used it on a course called "Digital Systems" in undergrad. It was one of my first CS courses - I had a project to create a digital calendar (it displayed time of day, day, month and year) and had so much fun learning to use it. Got to play with all sorts of quartz crystals, latches, flip-flops, etc.

In the end, it was so exciting getting it all working - I even implemented leap years (for february et al). Such a great tool to learn about circuit design!