Our quads do a couple of things, but they mainly straighten our knees when we stand up, which we do a lot in climbing. In contrast, our hamstrings mainly help bend our knees, which we do on occasion when high stepping or heel hooking. Exercising the hamstrings is kind of tricky because most specific sport specific movements initiated by the hamstrings are closed chain movements (our legs aren't usually moving, think about pulling off of a heel hook. The heel stays in place while the rest of the body moves). There is only one closed chained exercise for the hamstrings that I know, the Romanian Deadlift. This lift is great because it utilizes a barbell so the weight can increased until there just isn't anymore room. On the downside it doesn't look like anything that would be done in climbing. My favorite way to target the hamstrings is the Stability Ball Hamstring Curl. This exercise isn't closed chained but probably the closest thing to actually heel hooking and because it is performed on a stability ball it engages the core muscles as well.
The finger flexors and extensors reside in our forearms and they do exactly what the say they do. So when we are climbing we're flexing our fingers in order to grab on to the rock but we don't really extend our fingers forcefully. If our fingers are always flexing we're putting a lot of strain on the extensor muscles and tendons. It used to be that the main way to exercise the finger extensors was by doing Reverse Wrist Curls. I'm not a big fan of this method because in order to grip on to the dumbbell the flexors muscles would have to be engaged too (this is kinda like taking 2 steps forward and 1 step back, you're still headed towards the goal but very slowly). Luckily Metolius Climbing has developed the Grip Saver Plus.** The Grip Saver Plus is nice because the fingers can be extended fully as well as the wrist. If you don't want to fork out the money to buy a Grip Saver, you can always use a rubber band in the same fashion.
I feel that taking the time to train our antagonistic muscles is worthwhile, not only do these exercises help prevent injury, they give us something to do on our active rest days. Like always, if you have a question about anything discuss in this post or any of the others just let me know.
**I was not paid to mention the Grip Saver Plus**