Software Defined Radio is a technology which has always intrigued me, but something I just haven't been able to devote a lot of time to yet. I've built an early model SoftRock and studied the basics of the theory. Not too long ago, I built up my own little version of a 40 meter SDR based on a different topology from the SoftRock (VXO feeding a quadrature hybrid, driving passive MOSFET mixers). It worked so-so, and it ended up in my pile of experimental boards.
Today in my feed reader, I caught wind of a really neat topology for a minimal parts count SDR, courtesy of Joachim, DL1GSJ. His receiver uses a simple phase shift network to create the I/Q LO signals to drive a pair of Polyakov (subharmonic) mixers. It looks dead simple to breadboard and could be an easy way to try SDR without having to order a SoftRock kit. Get over to Joachim's blog to get the full details of this fascinating design.