Some of my favorite exercises for ski-related explosiveness:
Lateral jumps over an obstacle. I prop a broomstick (without broom) on a bench on one side and a caution cone on another. It goes from about 16" high behind to over 24" high in front. Then I jump back and forth over it. There's a balance/aiming/ landing component along with explosiveness training, since you can recover from jumping too far to the side etc. by jumping a little backward to make the next clearing the obstacle easier, and there's an aiming component, since any contact with the broomstick will knock it off the cone. The leg retraction to clear the broomstick is similar to leg retraction between slalom or GS turns. Each set is as many times as you can clear the broomstick, with rest between sets or a less anaerobically taxing recovery exercise. (An alternative is lateral cone hops, or jumping over an invisible obstacle.)
Box jumps or lateral box jumps. I use a roughly 16" bench or box to jump up and down from. Lateral jumps are more skiing specific (alternating sides) but slightly more difficult to get right as you tire, so jumps forward onto a box and backward down are better for anaerobic threshhold training, gradually increasing the number of repetitions you can do as your wind permits. The U.S. ski team (in my recollection from seeing it somewhere) wants its atheletes to progress to being able to do 90 seconds of uninterrupted box jumps (probably on a higher box than the rest of us use).
Weighted jumps. Take 25-30% of the weight you can squat, and hold two dumbbells that together total that weight. Do sets of 8 jumps, while holding the dumbbells low and outside your legs. Don't bang them into your knees. Men's Health recently mentioned some study showing that this exercise was a great way to increase vertical leap, a proxy for explosive leg strength.
Knee-tuck jumps. Sets of 30-40 jumps (you may have to work up to that number), where at the apex the legs are retracted and the knees are brought high to touch the hands held at a level just below your chest. Again, the retraction is similar to leg retraction during transition between ski turns, and also works the abs.
Lateral bounds. Basically, side-to-side jumps landing on the outside foot, compressing, and then exploding to repeat in the opposite direction.
Scissor jumps. Starting in a low lunge position, explode upward, landing in a lunge on the opposite leg and repeat, cycling so each alternating landing is in a lunge position with an opposite front leg. Be careful not to bang your knee into the ground.
Radcliffe's book, High Powered Plyometrics describes and pictures most of these, as well as a lot of other plyometric jumping exercises (as does Chu's Jumping Into Plyometrics. You can find both, along with Brown, Training for Speed, Agility and Quickness, and Musnick, Conditioning for Outdoor Fitness, on Amazon.com.) Radcliffe's book even has a specific progression of recommended plyometric exercises for downhill skiers at the end. My caveat on his recommended progression is that his set recommendations are way, way too short, and on the whole seem to concentrate too much on short sets of too many different exercises. Skiers who do rotational tricks can also try some of the exercises that involve twists. (Like hopping laterally over a cone, but finishing each alternating jump facing the opposite direction, doing a 180 degree turn in the air.)
On any plyometric exercise, make sure you do it on a surface with a little give to it. (Like, for example, grass. If your gym has just a carpet or rubber mat over concrete, that's probably not enough give. Add a cushioned exercise mat: Otherwise it's too hard on ankles and other connective tissue.)
In addition to being great for explosiveness training, plyometrics are great for skiers because (1) like skiing, they emphasize the eccentric (contraction) phase as you land and temporarily subject your muscles to extreme loads as they brake your momentum, (2) when used in larger sets, also are useful for anaerobic threshhold training, which seems relevant since skiing (well, skiracing or bump skiing, anyway) is an anaerobic, not aerobic sport, and (3) some large scale studies have shown that plyometric training is effective in preventing ACL tears in atheletes, especially female atheletes, who are 3 times as likely to suffer ACL tears as their male counterparts.
(not a physical therapist or certified anything, but with a lot of practice now in getting his 46 year old sea-level self in shape for ski racing...)