Low Tech Magazine has two very informative articles on using peddle power. The suprising conclusion, forget adding something to a bicycle and go with a flywheel system instead.
First article: http://www.lowtechmagazine.com/2011/05/bike-powered-electricity-generators.html
Second article: http://www.lowtechmagazine.com/2011/05/pedal-powered-farms-and-factories.html
Lots of pictures and diagrams for the DIY people.
We trash-picked an exercise bicycle with a front flywheel awhile back. We're planning to connect it to to our hand-crank grain mill, now that we've purchased a flywheel crank for the gain mill to replace the hand crank it came with. It may be a month or three before we get around to this project (we need to build a garden shed first), but we have the plans and it will happen. The grain mill is to my mind the best application for pedal power in an urban household that wants to have the freshest ground grains and flours. Hand cranking is pretty laborious in my experience, especially when attempting to grind wheat finely enough to make a good bread flour.