Bicycle Commuting: Trunk bag
This is a Topeak MTX DXP trunk bag. If you use their racks, the whole thing slides and clicks into place. There are velcro strips on the bag you can use to secure it to racks from other companies. The side pockets contain pull-out panniers. Quite a roomy trunk bag, this is.
When you remove it, you can use the included strap to carry it, and it feels like a camera bag.