I've previously enthused about the knobbly massage balls you can get from the physio and I still love and recommend them. The Bak Balls are a new discovery for me and I am using mine post-class and also just before bed.
Not just for my back - though placing it against a wall and rolling up and down has worked out some awful tension! Though it IS awkward, I can position myself over the ball on the mat and work it into my hip flexor to alleviate some of the arthritis-associated pain. I can also sit, angled to the side on it, and work into piriformis and glute medius (side of the butt!)
The original is more firm and comes in black. I have the softer version, which comes in red.
Get yours from Bad Backs