The mobile robotic drive units, or bots, roll through warehouses at 3 miles an hour, using barcodes on the floor to help them feel their way around. And guess what? Their putting warehouse workers out of work, because they're more efficient, don't take coffee breaks, and can lift half a ton. Here are four companies already rolling with bots:

1. Walgreens

walgreens_logo.jpgThe Mt. Vernon, Illinois facility uses more than 1,000 droids to fulfill orders. Merchandise is stored in central pods and when an order comes in, human operators dispatch the bots to pick up the appropriate pod containing the item ordered. When the bot brings the pod, the operator then finds the item he or she needs, and the bot takes the pod back to its storage location.

2. Zappos

Zappos_Logo.gifIn Shepherdsville, Kentucky, 72 bots roam the 832,000 sq ft Zappos warehouse organizing and delivering shelves of goods to operators, who then pack the items into boxes for shipping. The bots are also in charge of shifting the inventory.

3. Staples

staples.jpgAt Staples' Chambersburg, Pa., warehouse, the bots have helped operators triple their productivity, roaming the 500,000 sq ft facility much the same way they do at Walgreens, fetching centrally stored pods. One of the big draws of the bot-system, which is made by Kiva Systems, is that it allows companies like Staples, which is always changing its inventory, to be flexible. The whole warehouse can easily be reconfigured to accommodate changes or the need to store a new range of products.

4. The Gap

Picture 36.pngUntil now, the Gap has relied on sorter technology to increase order-picking efficiency. But the many undesirable side-effects, including a lot more end-to-end merchandise "ňútouches,' and big-time bottlenecks when the conveyor belts fail, have motivated them to switch to bots, too. With their exploding e-commerce sales (they now allow you to buy stuff from Old Navy and Banana Republic, too), they've decided it's time to lean on the droids to help fill orders.