Susette,
As Cambronne said, Performance offers good stuff. If you want some additional options, check Arkel at http://www.arkel-od.com and Lone Peak. Arkel offers a wide range of panniers from big touring bags to models designed specifically for commuting, like pannier backpacks and pannier briefcases, as well as a waterproof model. I am considering something they call a Utility Basket pannier. Basically just a big bag. Right now I have a largish (formerly handlebar) basket bolted to my rack. I like being able to just throw my backpack in it and bungee it down. I also have one folding saddle bag type basket. I am considering the pannier to replace the top basket/backpack in order to lower the center of gravity for more stability.
By the way, I have a Blackburn Expedition 2 rack. It is new, not even shown on their website at http://www.blackburndesign.com. Looks like the ER1, but with 4 vertical supports instead of 3. I tend to go for heavy duty stuff if the weight penalty is not great.
Oh, Bike Nashbar has their own brand of panniers and such also.
Regards,
Raymond