Free real-time logic simulator
Its' named Atanua (according to documentation I read, the name comes from Polynesian mythology, where Atanua is the goddess of dawn.)
In the following link you can see a demo.
There is a free version for personal, noncommercial use that you can get here:
The coolest thing is that there is a Linux version (compiled on Ubuntu 8.04), for Mac and Windows.
I already downloaded my version (for Linux, of course) and I'm going to play with it to see if it is as good as it seems on demo.
Do not waste time. Download and install the simulator and start playing / learning.