Mathlets
The MIT Mathlets are a set of small applets that dynamically illustrate various mathematical concepts. They can all be found at mathlets.org/mathlets
Here are direct links to some of the applets --MIT mathlets and others-- referenced in 18.031.
Amplitude Phase Second Order I
Amplitude Phase Second Order II
Bode and Nyquist Plots
Amplitude Response: Pole Diagram
Series RLC Circuit
LRC Filters
Mascot
Here is a description of the Course Mascot which we will introduce in class.
And here is a mathematical account of this kind of feedback mechanism.