I have a wald basket like wahoonc mentioned, the only difference is mine is the folding model. It comes with the mounting hardware to secure it to any rear rack, folds flat when not in use and is not noisy like I would have expected. It will carry something about the size of a paper grocery bag and you can mount them on both sides. With these side baskets I can strap something to the top of my rack for more storage if I need it.
__________________
|^^^^^^^^^^^^^^| ||
|......GO.BROWNS........| ||'|";, ___.
|_..._..._______===|=||_|__|..., ] -
"(@)'(@)"""''"**|(@)(@)*****''(@)