If you have a Backpack, you can bungee it to the top of the rack. I attached a milk crate to my rack and dump my backpack into it while riding. I can also throw in a few bags of groceries when I go shopping. A few DIY'rs have also made their backpacks into panniers.

Otherwise you I have the Axiom Kootenays. They are nice and big, but I don't use them much (would hate to have to keep taking them on and off). The other option I can think of is the Nashbar waterproof or ATB panniers.