Make your own metal detector with these easy to follow instructions.  The plans are for a BFO ( beat frequency oscillator ) detector.  This essentially works by have two oscillators close to the same frequency.  One is used as a reference, the other contains a wide "search" coil of wire.  They are wired such that you can hear the difference between the two oscillators.  That's the typical beep or chirp you're probably familiar with.  When passed over a piece of metal, the "search" osciallitor's frequency varies slightly.  That variation is pretty noticible via the audible "chirp".  Translated: that means there's some junk close by.

Most of the required parts can be salvaged from an old transistor radio.  If you do have to purchase them, it's pretty inexpensive.  Most of what you need are capaciters and resisters.  Of course you need some type of form factor too...can't just hold a handful of components over the ground.  The plans are for the electronics.  They do have a few pictures to give you some ideas though.

Their main site has some other handy information as well. 
   How to test a metal detector
   Make your own digging tool
   The treasure act of 1997
   Metal detector patents
   etc.

Build your own beat frequency oscillator metal detector here