REI Commuter Trunk Rack expands into small panniers. Yellow. Internal organizers. Used a dozen times but almost new. Asking $25 with shipping. Current REI price $69.50.

http://www.rei.com/product/780461

I found this handy biking to work in cool weather when I had lots of clothes to take off 20 minutes into the ride. But I generally carry my Ortlieb front panniers now and have another trunk rack. This bag is noticeably heavier than a standard trunk rack, but very functional for the volume.