Park Tool Derailleur Alignment Tool. We have found this one to be the best, and it comes at a fair price.
The DAG-2.2 is an indispensable shop tool that can be used to both measure and straighten misaligned derailleur hangers solving a high percentage of shifting problems
This is a simple, sturdy tool. It seems a bit pricey for what it does, but is reasonably priced relative to other manufacturers. Park Tool tends to be very reliable and long lasting.