I have had success with the Tap-Jump-Tap box drill (15sec on–45sec/off), as well a lunge off a box where you jump side to side on the same leg for awhile over a small object. Pistol squat progressions also seem very useful, both for climbing and skiing. Single-leg stuff seems best IMO.
Otherwise, the UA Box step-down is really great, as are leg blasters (aka quadzilla complexes), although with the latter I find it important to monitor form to prevent injury–its easy to get sloppy when tired. I have also evolved to just use weighted, jumping lunges instead of leg blasters because I think thats where the money is.
Single leg balance exercises like single-leg Romanian deadlifts are great. As are those one-legged pelvic lifts that train your hamstrings (also can be weighted). And of course the good old deadlift for posterior chain deficiencies.
One could also do negatives (eccentrics) on a lot of these exercises, or at least do the eccentric portions slow, which should provide a better descent-focussed work-out.
Hope this helps a bit.
