I have a milk crate (the ones that hold 4, 1 gallon jugs) zap strapped to my rear rack. Where I live, it's actually a fairly common choice. It's good for a few groceries, a load of library books, a 24 case of Pepsi, even my lunch box. I used to use panniers, but found the crate to better meet my needs. Will look into modding it to hold a few more rear lights.

Dear Dlion:
I always use a plastic milk crate as it makes your bike look cheap and has less interest from thieves. I know there is more air drag, but we are commuters; not racers!

Panniers cost money, the way to go is a kittier, i.e. a bucket for your bike rack, made of kitty litter containers.

You have to be willing to look like a weirdo, like me:

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But on the plus side, lots of surface area for decoration with reflective tape

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I keep on trying different things, but keep on going back to my Wald folding baskets, 582 Rear Folding Basket - Waldsports They're hard to beat for do-it-all versatility.

I've used many different methods, including the plastic milk crate. The crate is cheap, and it's very convenient, because it holds anything. Drop in whatever bag you have with you. You can go to the supermarket without a bag, they'll give you a bag for your stuff, and you can drop that in.

It's heavier than other methods, and it puts the weight high, which can affect handling of the bike, though not terribly.

Currently, I use panniers. They're more expensive, but they're a lot lighter than a crate. When I'm not carrying anything, the only excess weight I have is the rack. When I used a crate, it was inconvenient to remove the crate, so I was pedaling around both a rack and a crate.