When fishing in catch and release areas, one needs to be able to unhook the fish and return them to the water unharmed.
What tool(s) would be useful for this?
The best tool I have found for this is a Hemostat.
They are long and thin, enabling you to reach in and get to fishhooks that are deep inside and can be locked onto the hook.
It's also really easy to carry them when fly fishing, you simply clamp them onto your fishing vest and then they can be opened with one hand.
These are way better and lighter than plyers and
Charlie Brumbaugh's answer is definitely the way to go for smaller fish such as panfish or trout. However, for bigger fish such as northern pike or musky I go with a longer (12-16"/~30-40 cm) pair of needle nose pliers as seen below.
Photo source: www.ebay.com
I prefer to use either the middle option, with the slight bend, or straight option on the right because the more curved option on the left can put your wrist in an awkward position if they decide to try to roll. Many times I'm able to just reach over the side of the boat with these and never even have to touch the fish. As an added bonus I do not need to get my fingers anywhere near their teeth.