If you want to explore these Virtual Labs more at home,
their URLs are:
http://www.jhu.edu/~virtlab/logic/log_cir.htm
for the JHU Logic Circuit Builder at Johns Hopkins University (Baltimore, MD),
and:
http://math.hws.edu/TMCM/java/labs/xLogicCircuitsLab1.html
For the xLogicCircuits, "Lab 1: Logic Circuits", at Hobart and
William Smith College (Geneva, NY).