When I was a kid, I had a great fascination with electronics – as in how circuits work, not buying consumer electronics. One thing that would have really helped back then would have been a computer program to simulate circuits.
These applications did exist. I can remember seeing an ad for one of them, but they tended to be ludicrously expensive.
Today, I found Logisim, which does exactly what I would have loved to have had 15 years ago, and released as Open Source.
What Logisim does, is allow you to draw logic circuit diagrams with built-in components, including logical gates, timers and other components, and then also run tests on the circuit to see the results.
This app is useful for people ranging from the learning logician to the next computer designer.