The Insurance Institute for Highway Safety, the same organization that does crash tests, has come out with new ratings for dozens of children's booster seats. The key measure was how the seats do what they're supposed to: position children properly for adult-sized seat belts.
The critical things to look for, according to the IIHS:
-The lap part of the belt "should fit flat across a child's upper thighs, not the soft abdomen"
-and the shoulder part of the belt "should cross snugly over the middle of the shoulder."
Here's a list of the 9 models that did best.
Here's are 6 more rated as good.
And here are 11 rated "not recommended."
