David Cox created a GeoGebra applet to help students learn about x and y intercepts and posted it on his blog, calling it the un-lecture. Students work through a series of randomly generated equations of lines, and learn how to find x and y intercepts on their own.
His post describes the applet, includes a pre- and post-test great formative assessment tools), and explains what his applet is all about.
As an aside, I took David’s applet, and with a lot of his support was able to build my own that is similar, but says slightly different things. I learned a lot about scripting in GeoGebra by going through that. Here’s the one I built. John’s Variation.