Logik is an easy-to-use and user-friendly logic circuit simulator. It is suitable not only for creating circuit prototypes, but also for use in education.
For a hands-on tutorial and a quick reference, see:
The program is not under active development, but it is very much functional (it works on my machine :)). I use it sometimes for school.
The internal code however leaves a lot to be desired and a large part of it would probably need to be rewritten if a bigger upgrade is needed.