One of these....
http://www.amazon.com/Park-Tool-DAG-...erailleur+tool
Park Tool DAG-2.2 Derailleur Hanger Alignment Gauge
Ouch... $70.
I saw a video of how they can straighten out the derailleur mechanism if it's too close to the spokes.
If you want to adjust brakes, it help if the wheel is true. It's all building on the next layer down.
If I want to adjust shifting... Would I use this tool to make sure everything is correct at a more basic level first?
My shifting works. Maybe it's not perfect. I should figure out how to post videos somehow.... I can add that my bike has tipped over a few times. Considering I've had it maybe seven years now, that's tipping over many times I would think. So someone will probably say yes I need the tool, is my guess.
(I haven't watched the Park Tool video yet. I will. It's on my list.) I've been tool hunting a bit today.