Drop sets have been around since the invention of the dumbbell. The concept is ridiculously simple but effective nonetheless. Train to exhaustion, then immediately reduce the weight to perform more reps. Continue until total failure, or until you crap a kidney.
Obviously, dumbbell exercises like lateral raises or biceps curls work exceptionally well with this method, as do exercises using a selectorized pulley such as triceps pressdowns. Barbell exercises also work if you have a partner to help you slide off the plates.
A good rule of thumb is 20% of your work weight for a given set per drop. So if you're using 200 pounds on an exercise for 8 reps, you'd drop the weight by 40 pounds, or 20 pounds per side. This should allow you to perform another 3-4 reps, depending on the exercise, your training age, and your muscle fiber make-up.