If your electricity use is average (around 11,500 kWh/year for US households) and you lived somewhere with 30 mph winds 24-7, this thing could provide about 7.5% of your power needs. Nobody I know lives anywhere that is consistently that windy, but the Chispito Wind Generator could be a fun and educational DIY project in a lot of places. Here is an 80-meter wind resource map of the US that may give you some idea of the wind speeds in your area, although 80 meters is a bit high for a back yard wind generator.
For pictures and build instructions, check out "How to Build a Chispito Wind Generator" page.
You make the blades on this baby using some cut up PVC pipe and sand paper. I love that the tools and materials needed to build these blades are so close to universally accessible. If you follow this blog, bets are probably good that you could finish a significant percentage of this wind power generator project before making a trip to the hardware store.
All that said, the DIY aviation nut in me is screaming that we need an inexpensive, open-source way to optimize airfoils for domestic-use, DIY wind power generators. Look out for a future post on the huge increases in power generation that can be obtained by using well-optimized airfoils, and my ideas about how we can make such airfoils inexpensive and widely available.