Probably one of the cleverest things I've ever built. It only took half an hour or so to make, and is quite simple, but it's well designed and super useful. And it's exactly what it looks like: two milk crates hanging off the back of a bike. The best part about it is it's ease of use. It takes about 5 seconds to put on, and 5 seconds to take off, without exageration or even practice. I've spent a fair bit of time using regular panniers, and here's why these milk-crate panniers are better for going out and getting groceries:

• They take less time to put on and take off
• No zippers or clips in the way
• Don't squish your food
• Just plop your grocery bags in and you're done loading; no special packing needed
• Not as big of a deal if it gets stolen
• Makes fun noises on bumps
• Cheaper, as in free

There are no clips or anything to attach it to your rack. You sort of just drop it on. My biggest load to date has been 22lbs of flour on one side and a boxed set of 14 glass cups on the other. It rode just fine. And it doesn't fall off for the following reasons:

• Can't go side to side because the milk crates are squished up against the bike's rack on both sides
• Can't go forwards and backwards because four screws on the bottom side of the wooden crossbar are stradling a crossbar on the bike's rack. See the picture
• Can't go up and down because of gravity

Construction notes: Just 12 screws, a stint of 2x4, and two milk-milk crates. Make sure you find the kind of milk-crate with rectangluar handles. Some of them have trapezoidal handles, and the 2x4 won't fit in. And then just make a wooden "U", with four appropriately placed screws screwed into the bottom. I wrapped them in duct-tape so that the threads wouldn't scratch my rack. The photo shoot is here: