You probably have an old set of corded phones sitting in a box on a shelf somewhere. This fun little project, which is great for doing with kids, turns a pair of those corded phones into a DIY intercom.
As DIY blog Make points out, all the circuitry for processing voice is already in the phones. To connect them, you just need a 9V battery and a resistor. Check out the video above, and then hit up the full build over at Make.