Two that I find great for strength and injur prevention.
First, back extensions, lie on your stomach with arms forward and lift both simultaneously.
Second, think they are technically known as reverse crunches or gorilla press. Lie on your back, draw your legs up to your chest keeping your back on the floor.
These two have improved my core strength and got rid of most of my back problems.
Other great ones...
Plank- press-up position, but on elbows/arms, and just hold it for about a minute
Leg raises.