RepRap

Not RF related, but homebrew of the finest kind.

RepRap

The goal of the RepRap project is to build a machine
that can reproduce itself. RepRap has recently selected
the Arduino to be the computer building block of choice.

RepRap on Wikipedia

Sources of parts for home-brew

Sources of Electronic Mechanical and Surplus Parts is an
extensive list of links to suppliers of goodies that
can be used in home-brew projects.

Part of an interesting site on classic electronics kits:
Nostalgic Kits Central