Deadlift, stiff-leg deadlift, good-morning, and secondarily on things like bent-over row. Links with animations:
Barbell DeadliftBarbell Stiff Leg DeadliftBarbell Bent Knee Good-morningGood morning is one people don't do much, and it's great. Get some weight on your shoulders (normally by taking a barbell, as you would for a squat). Your back should be "set" - abs in, shoulders back, proper like. Now bow from the hips, as if you were a butler formally bowing to your master (I always say "Good morning, Master Bruce" with each rep because I am (1) a colossal nerd and (2) it keeps the rep nice and slow; YMMV but it's not one to rush). Start this exercise light because if you're back's wrong you want to feel "uh-oh" not "ARGH" -- an empty bar for you virgin set.
Good mornings are pretty lower-back specific.
Stiff-leg deadlift (like Vicky there) is probably the most all-round useful as you can get some really good bum and upper back work, traps, grip, all sorts, especially if you shrug the bar back at the top of each rep.
Regular deadlift is able to handle lots of weight. If on any of these you find your grip going, switch to an alternate grip. One hand is knuckles-out, the other knuckles-in, and you'll find you can hold a fair bit more.