This java applet is an electronic circuit simulator that displays the frequency response of circuits. When it starts up, it displays a simple high-pass filter. You can adjust the cutoff frequency using the slider on the right. The "Circuits" menu contains a lot of other sample circuits for you to try.
If you don't have Java, get the Java plug-in.
Main circuit applet.
Zip archive of this applet.
(Version 0.91, posted 4/26/14)