I really like these REI/Novara Panniers and what's better is that right now they're on sale $34 each (down from $50 each). They are quite sizable in volume, very stable on the rack, and the built in handles allow you to use it just like a reusable grocery bag so that you can save the paper/plastic. It has an open top but is equipped with a buckle strap to cinch the top shut if you don't want things bouncing out. Also it folds up very nicely when not loaded. I have an older version which is basically the same except that they raised the position of the hooks so that it rides lower on your rack (better for stability and use of the top of the rack) and the older version didn't have the strap across the opening.
http://www.rei.com/product/825299/no...e-bike-pannier