In the same vein, there is all the great stuff at play-hookey.com
Also, there's the Circuit Simulator Applet
. This allows you to examine a library of circuits with a really neat visualization of current, as well as create your own circuits. Logic gates here are covered in both a low-level and high-level. You can either see them as diode-resistor logic, transistor-transistor logic, CMOS, etc components, or in their abstracted form. Yet another tool you cannot go without when trying to understand this stuff.