This project is possibly the simplest induction-balance type detector you can build. Add to that a very detailed construction article and you have an excellent beginner project. Might also be a good probe circuit.
This article appeared in the June 2002 issue of Silicon Chip, an electronics magazine out of Australia. Silicon Chip is especially nice in that project articles are published in full color, with nicely designed graphics. I've post the PDFs in both color and B&W, each with both 150dpi and 300dpi resolution.