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.

Full Screen version.

Standalone (offline) versions.

Main circuit applet.

Huge thanks to Iain Sharp for the Javascript port. You can still use the original Java version. More acknowledgements in the about box.