M. and H. turned me on to this great basket from Nashbar, and, when I ordered it, the versatile M. and H. even installed it for me. It's quick-release, so perfect for trips to the co-op. You gather up your groceries in there, check out, then click the basket onto your bike, and you're off.
In related news, I was offered an employee parking space--at the ripoff price of $46/month--and I declined! My basket and I can take on the (~1 mile) commute!