I just got this a couple days ago and thought I'd share because I love it! The bag is by Basil. It has two plastic hooks that hook onto your rear rack, and the bag comes off and you can carry it around as a tote.
It's not strong or heavy enough to be used like a pannier, but I used it for a trip to the farmer's market yesterday and for lightweight stuff it's perfect. I've ordered a second one for the other side of my bike!

