Explosive strength is one of the most sought after athletic components for virtually every sport, and cross fit is no exception. Producing force quickly is monumentally important for the Olympic lifts, sprinting, squatting, kipping, jumping, and many other things. There are many different ways to build this but one of the best ways is to jump high and jump often in many different ways. Keep in mind the most important thing is jumping high, or far, so short box jumps will not further you jumping ability.
Barbell jump squats are great to create explosiveness under a load, however i would keep volume low and use no more than 50% 1RM.
http://www.allthingsgym.com/weighted-barbell-squat-jumps/
The Single leg box jump is one of my favorites because of the unilateral stability and transfer to sprinting, Start low and work your way up.
Two other Great exercises are single leg bounding and drop jumps,really focus on the distance of jump on the bound, and focus more on the rebound of the drop jump.
http://www.allthingsgym.com/lucas-jakubczyk-100m-sprinter-jumps/