In the same vein, there is all the great stuff at
play-hookey.com. There are some very well-done Javascript simulations of logic gates, ripple counters, shift registers, etc. Extremely useful when trying to understand how they work.
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.