If you've got four speakers and don't want to lash out on a new surround sound system (or 'er indoors refuses to have any more AV equipment in her living room!), try this Hafler circuit.
I first found this in Everyday Electronics in 1975 (thanks guys!), and have been using various systems (once in the car) ever since.
It's not a 7.1 system, but it will add hugely to your enjoyment of stereo - and especially to DVD film soundtracks. Plus it doesn't matter where you sit in the room - you always hear stereo.
Step 1: Warning
I have heard that this system can overload your amp.
I have been using this circuit on a range of hifi equipment for thirty years, and I've never had a problem. But continue at your own risk.