Pretty much ditched my car for a bike, a few months ago. Started researching ways to carry more stuff. The XtraCycle Free Radical attaches to the back of your bike where your rear wheel used to be. Then you take your rear wheel and attach it to the Free Radical. If you want to see more about this, you can click on the link in the title.
As you can see from the picture, you can load it up with a lot of groceries- up to 200 pounds worth! Recently met a Spokane family that has had one of these for years, and swears by them.