freestompboxes.org

http://www.speech.kth.se/smptool/
especially the Tombstone...

With the Tombstone you can plot the actual frequency response of your circuit, a great tool to examine the tone controls, cab simulators etc. I tried it with my px4d, here are some frequency responses of the cab simulators:
(at the end of the page)
Link
Another example: a typical freq response of the 4 stage phaser (no feedback):