You can do some pretty cool stuff on this site. It has built-in tools for drawing a schematic, running simulations, and viewing the results.
Try this example
http://www.systemvision.com/design/pid-speed-control-loop-switching Click on "view design" and move the probes around. You can even probe inside the different symbols (like see the current inside a resistor).
You can create a free account and copy the design and edit it.