Back-up

Does anyone know any good lower back exersices?

deadlifts deadlifts and more deadlifts. you'll strengthen your lower back along with a dozen other muscles all at the same time.

Hard to say more when Borreck already said Deadlifts. Here are some others, that include Deadlifts.

http://www.bodybuilding.com/fun/exercises.php?MainMuscle=Lower+Back

On the site you showed, Dereck, they have stiff-legged deadlifts, here is an example a deadlift done bending the legs:
http://www.exrx.net/WeightExercises/ErectorSpinae/BBDeadlift.html

Are either of the two better than the other? Advantages?Disadvantages?

The one you posted Joel is the actual one I do with the legs bending. This then allows the legs to get worked more then the stiff legged ones on the site I showed. I believe they both serve their purpose and have advantages and disadvantages but I couldn't say for sure as I don't think I'm qualified enough. Thanks for posting yours as well.

Normal deadlifts incorporate the quads into the lifting of the weight, stiff legged deadlifts lift purely from effort in the lower back and hamstrings.
Stiff leg deadlifts do place a greater stress on the lower spinal discs due to greater load concentration on the area, this makes cranking the weight up, and not using perfect form far more likely to injure you.

Other lower back exercises.

Squat: the answer to everything works the lower back like a charm.

Good mornings: light barbell across upper back/neck (same position as squat). Legs 'soft' at knees. Bend forward from the waist/hips, keeping the spine straight till your torso is nearly parallel to the floor. Now stand up straight. Got its name as it looks a bit like someone bowing when saying 'good morning' back in the days of polite society

Hyperextension. Can be done on a hyperextension station in a gym, or can be done at home as an exact opposite movement to a weighted crunch.
Lie face down on ground, hold a weight steady on your upper traps, now arch your back, raising your rib cage off the ground as high as possible (do not crane your neck- just keep looking forward and slightly down). Hold at max contraction, then down and repeat for target reps.

Havent a name for these next ones. Not sure if I invented them, but i have not seen them done elsewhere.

lie face down on a flat weights bench, with your lower body (hips and legs) hanging off the back of it (you will need to hold the bench with your hands in line with your head).
Your position is now like you are bent over the bench, with your knees on the floor. By tensing your hips and locking in proper posture, you will raise the knees slightly off the floor. Keeping your feet together do a slow double 'back kick' straightening the legs as you thrust the feet back so that the whole body is now parallel with the floor, pause, and then 'tuck' your legs back in to the start position. When you get the hang of it, add a dumbell held between the feet.

They should keep you busy